9 Visual formatting model

9.1 Introduction to the visual formatting model

This chapter and the next describe the visual formatting model: how user agents process the document tree for visual media.

In the visual formatting model, each element in the document tree generates zero or more boxes according to the box model. The layout of these boxes is governed by:

• box dimensions and type.
• positioning scheme (normal flow, float, and absolute).absolute positioning).
• relationships between elements in the document tree.
• external information (e.g., viewport size, intrinsic dimensions of images, etc.).

The properties defined in this chapter and the next apply to both continuous media and paged media. However, the meanings of the margin properties vary when applied to paged media (see the page model for details).

The visual formatting model does not specify all aspects of formatting (e.g., it does not specify a letter-spacing algorithm). Conforming user agents may behave differently for those formatting issues not covered by this specification.

9.1.1 The viewport

User agents for continuous media generally offer users a viewport (a window or other viewing area on the screen) through which users consult a document. User agents may change the document's layout when the viewport is resized (see the initial containing block).

When the viewport is smaller than the document's initial containing block,area of the canvas on which the document is rendered, the user agent should offer a scrolling mechanism. There is at most one viewport per canvas, but user agents may render to more than one canvas (i.e., provide different views of the same document).

9.1.2 Containing blocks

In CSS2,CSS 2.1, many box positions and sizes are calculated with respect to the edges of a rectangular box called a containing block. In general, generated boxes act as containing blocks for descendant boxes; we say that a box "establishes" the containing block for its descendants. The phrase "a box's containing block" means "the containing block in which the box lives," not the one it generates.

Each box is given a position with respect to its containing block, but it is not confined by this containing block; it may overflow.

The root of the document tree generates a box that serves as the initial containing block for subsequent layout. The width of the initial containing block may be specified with the 'width' property for the root element. If this property has the value 'auto', the user agent supplies the initial width (e.g., the user agent uses the current width of the viewport ). The height of the initial containing block may be specified with the 'height' property for the root element. If this property has the value 'auto', the containing block height will grow to accommodate the document's content. The initial containing block cannot be positioned or floated (i.e., user agents ignore the 'position' and 'float' properties for the root element). Thedetails of how a containing block's dimensions are calculated are described in the next chapter.

9.2 Controlling box generation

The following sections describe the types of boxes that may be generated in CSS2.CSS 2.1. A box's type affects, in part, its behavior in the visual formatting model. The 'display' property, described below, specifies a box's type.

9.2.1 Block-level elements and block boxes

Block-level elements are those elements of the source document that are formatted visually as blocks (e.g., paragraphs). SeveralThe following values of the 'display' property make an element block-level: 'block', 'list-item', 'compact' and 'run-in' (part of the time; see compact and run-in boxes ),and 'table'.

Block-level elements generateboxes are boxes that participate in a principalblock formatting context. Each block-level element generates a principal block-level box that onlycontains block boxes . The principal block box establishes the containing block fordescendant boxes and generated content and is also the box involved in any positioning scheme. Principal block boxes participate in a block formatting context .Some block-level elements may generate additional boxes outside ofin addition to the principal box: 'list-item' elements and those with markers .elements. These additional boxes are placed with respect to the principal box.

Anonymous block boxesExcept for table boxes, which are described in a document like this: <DIV> Some text <P>More text </DIV> (and assuming the DIV and the P both have 'display: block'), the DIV appears to have both inline contentlater chapter, and replaced elements, a block-level box is also a block content.container box. A block container box either contains only block-level boxes or establishes an inline formatting context and thus contains only inline-level boxes. Not all block container boxes are block-level boxes: non-replaced inline blocks and non-replaced table cells are block containers but not block-level boxes. Block-level boxes that are also block containers are called block boxes.

The three terms "block-level box," "block container box," and "block box" are sometimes abbreviated as "block" where unambiguous.

9.2.1.1 Anonymous block boxes

In a document like this:

<DIV>
  Some text
  <P>More text
</DIV>

(and assuming the DIV and the P both have 'display: block'), the DIV appears to have both inline content and block content. To make it easier to define the formatting, we assume that there is an anonymous block box around "Some text".

In other words: if a block container box (such as that generated for the DIV above) has another blocka block-level box inside it (such as the P above), then we force it to have only blockblock-level boxes inside it, by wrappingit.

When an inline box contains an in-flow block-level box, the inline box (and its inline ancestors within the same line box) are broken around the block-level box (and any block-level siblings that are consecutive or separated only by collapsible whitespace and/or out-of-flow elements), splitting the inline box into two boxes (even if either side is empty), one on each side of the block-level box(es). The line boxes before the break and after the break are enclosed in ananonymous block boxes, and the block-level box becomes a sibling of those anonymous boxes. When such an inline box is affected by relative positioning, any resulting translation also affects the block-level box contained in the inline box.

 /* Note: HTML UAs may not respect these rules */ BODYp    { display: inline }
 Pspan { display: block }

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML  4.0 Transitional//EN">4.01//EN">
<HEAD>
<TITLE>Anonymous text interrupted by a block</TITLE>
</HEAD>
<BODY>
<P>
This is anonymous text before the  P. <P>ThisSPAN.
<SPAN>This is the content of  P.</>SPAN.</SPAN>
This is anonymous text after the  P.SPAN.
</P>
</BODY>

The properties of anonymous boxes are inherited from the enclosing non-anonymous box (in(e.g., in the example:example just below the subsection heading "Anonymous block boxes", the one for DIV). Non-inherited properties have their initial value. For example, the font of the anonymous box is inherited from the DIV, but the margins will be 0.

9.2.2 Inline-levelProperties set on elements and inlinethat cause anonymous block boxes Inline-level elements are those elements ofto be generated still apply to the source document that do not form new blocksboxes and content of that element. For example, if a border had been set on the P element in the above example, the border would be drawn around C1 (open at the end of the line) and C2 (open at the start of the line).

Some user agents have implemented borders on inlines containing blocks in other ways, e.g., by wrapping such nested blocks inside "anonymous line boxes" and thus drawing inline borders around such boxes. As CSS1 and CSS2 did not define this behavior, CSS1-only and CSS2-only user agents may implement this alternative model and still claim conformance to this part of CSS 2.1. This does not apply to UAs developed after this specification was released.

Anonymous block boxes are ignored when resolving percentage values that would refer to it: the closest non-anonymous ancestor box is used instead. For example, if the child of the anonymous block box inside the DIV above needs to know the height of its containing block to resolve a percentage height, then it will use the height of the containing block formed by the DIV, not of the anonymous block box.

9.2.2 Inline-level elements and inline boxes

Inline-level elements are those elements of the source document that do not form new blocks of content; the content is distributed in lines (e.g., emphasized pieces of text within a paragraph, inline images, etc.). SeveralThe following values of the 'display' property make an element inline:inline-level: 'inline', 'inline-table', 'compact' and 'run-in' (part of the time; see compactand run-in boxes ).'inline-block'. Inline-level elements generate inlineinline-level boxes . Inline, which are boxes maythat participate in severalan inline formatting contexts: Within a block box,context.

An inline boxesbox is one that is both inline-level and whose contents participate in anits containing inline formatting context .context. A compact inline box is givennon-replaced element with a position in the margin'display' value of a block'inline' generates an inline box. MarkerInline-level boxes that are also given positions outside of a block box. Anonymousnot inline boxes (such as replaced inline-level elements, inline-block elements, and inline-table elements) are called atomic inline-level boxes because they participate in their inline formatting context as a single opaque box.

9.2.2.1 Anonymous inline boxes

Any text that is directly contained inside a block container element (not inside an inline element) must be treated as an anonymous inline element.

In a document with HTML markup like this:

 <P>Some <EM>emphasized</em> text</P><p>Some <em>emphasized</em> text</p>

the P<p> generates a block box, with several inline boxes inside it. The box for "emphasized" is an inline box generated by an inline element (EM),(<em>), but the other boxes ("Some" and "text") are inline boxes generated by a block-level element (P).(<p>). The latter are called anonymous inline boxes, because they don'tdo not have an associated inline-level element.

Such anonymous inline boxes inherit inheritable properties from their block parent box. Non-inherited properties have their initial value. In the example, the color of the anonymous initialinline boxes is inherited from the P, but the background is transparent.

White space content that would subsequently be collapsed away according to the 'white-space' property does not generate any anonymous inline boxes.

If it is clear from the context which type of anonymous box is meant, both anonymous inline boxes and anonymous block boxes are simply called anonymous boxes in this specification.

There are more types of anonymous boxes that arise when formatting tables.

9.2.3 CompactRun-in boxes

A compact box behaves as follows: If a block box (that does not float and is not absolutely positioned ) follows[This section exists so that the compact box,section numbers are the compact boxsame as in previous drafts. 'Display: run-in' is formatted like a one-line inline box. The resultingnow defined in CSS level 3 (see CSS basic box width is compared to one of the side margins ofmodel).]

9.2.4 The 'display' property

The choicevalues of left or right margin is determined by the 'direction' specified forthis property have the following meanings:

block

This value causes an element producing the containingto generate a block for the compact box and followingbox.

If the inline box width is less than or equalinline-block

This value causes an element to generate an inline-level block container. The margin, the inline boxinside of an inline-block is given a position in the marginformatted as described immediately below. Otherwise, the compact box becomesa block box.box, and the compact boxelement itself is given a position in the marginformatted as follows: it is outside (to the left or right) of the first line box of the block, but it affects the calculation of that line box's height . The 'vertical-align' property of the compact box determines the vertical position of the compact box relative to that line box. The horizontal position of the compact box is always in the margin of the blockan atomic inline-level box.

inline

This value causes an element that cannot be formatted onto generate one line cannot be placed in the margin of the following block. For example, a 'compact'or more inline boxes.

list-item

This value causes an element (e.g., LI in HTML that contains a BR element will always be formatted asHTML) to generate a principal block box (assuming the default style for BR, which inserts a newline). For placing multi-line texts in the margin, the 'float' property is often more appropriate. The following example illustratesand a compactmarker box. <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN"> <HTML> <HEAD> <TITLE>A compact box example</TITLE> <STYLE type="text/css"> DT { display: compact } DD { margin-left: 4em } </STYLE> </HEAD> <BODY> <DL> <DT>Short <DD><P>Description goes here. <DT>too longFor information about lists and examples of list formatting, please consult the margin <DD><P>Description goes here. </DL> </BODY> </HTML>section on lists.

none

This example might be formatted as: short Description goes here too long for the margin Description goes here The 'text-align' property can be used to align the compactvalue causes an element inside the margin: against the left edge of the margin ('left'), against the right edge ('right'), or centeredto not appear in the margin ('center').formatting structure (i.e., in visual media the value 'justify' doesn't apply,element generates no boxes and is handled as either 'left' or 'right', dependinghas no effect on layout). Descendant elements do not generate any boxes either; the 'direction' of the block-level element in whose margin the compactelement is formatted. ('left' if the direction is 'ltr', 'right' if it is 'rtl'.) Please consult the section on generated content for information about how compact boxes interact with generated content. 9.2.4 Run-in boxes A run-in box behaves as follows: If a block box (that does not floatand is not absolutely positioned ) follows the run-in box, the run-in box becomesits content are removed from the first inline box offormatting structure entirely. This behavior cannot be overridden by setting the block box. Otherwise,'display' property on the run-in box becomes a block box. A 'run-in' box is useful for run-in headers, as in this example: <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN"> <HTML> <HEAD> <TITLE>A run-in box example</TITLE> <STYLE type="text/css"> H3 { display: run-in } </STYLE> </HEAD> <BODY> <H3>A run-in heading.</H3> <P>And a paragraph of textdescendants.

Please note that follows it. </BODY> </HTML> This example might be formatted as: A run-in heading. Anda paragraphdisplay of text'none' does not create an invisible box; it creates no box at all. CSS includes mechanisms that follows it. The properties of the run-inenable an element are inherited from its parentto generate boxes in the source tree,formatting structure that affect formatting but are not from the block box it visually becomes part of.visible themselves. Please consult the section on generated contentvisibility for information about how run-in boxes interact with generated content. 9.2.5 The 'display' property 'display' Value:    inline | block | list-item | run-in | compact | marker |details.

table |, inline-table |, table-row-group |, table-column, table-column-group, table-header-group |, table-footer-group |, table-row | table-column-group | table-column |, table-cell |, and table-caption

| none | inherit Initial:    inline Applies to:    all elements Inherited:    no Percentages:    N/A Media:    all TheThese values of this property have the following meanings: block This value causescause an element to generatebehave like a principal block box. inline This value causes antable element (subject to generate one or more inline boxes. list-item This value causes an element (e.g., LIrestrictions described in HTML) to generate a principal block box and a list-item inline box. For information about lists and examples of list formatting, please consultthe sectionchapter on lists . marker This value declares generated content before or after a box to be a marker. This value should only be used with :before and :after pseudo-elements attached to block-level elements. In other cases, thistables).

The computed value is interpretedthe same as 'inline'. Please consultthe section on markersspecified value, except for positioned and floating elements (see Relationships between 'display', 'position', and 'float') and for more information. none This value causes an element to generate no boxes inthe formatting structure (i.e.,root element. For the element has no effect on layout). Descendant elements do not generate any boxes either; this behavior cannot be overridden by settingroot element, the 'display' propertycomputed value is changed as described in the section on the descendants. Pleaserelationships between 'display', 'position', and 'float'.

Note that a displayalthough the initial value of 'none' does not create an invisible box; it creates no box at all. CSS includes mechanisms that enable an element to generate boxes'display' is 'inline', rules in the formatting structure that affect formatting but are not visible themselves. Please consultuser agent's default style sheet may override this value. See the section on visibility for details. run-in and compact These values create either block or inline boxes, depending on context. Properties apply to run-in and compact boxes based on their final status (inline-level or block-level). For example, the 'white-space' property only applies if the box becomes a block box. table , inline-table , table-row-group , table-column , table-column-group , table-header-group , table-footer-group , table-row , table-cell , and table-caption These values cause an element to behave like a table element (subject to restrictions described in the chapter on tables ). Note that although the initial value of 'display' is 'inline', rules in the user agent's default style sheet may override this value. See the sample style sheetsample style sheet for HTML 4.04 in the appendix.

p   { display: block }
em  { display: inline }
li  { display: list-item } 
img { display: none }      /*  Don'tDo not display images */

Conforming HTML user agents may ignore the 'display' property.9.3 Positioning schemes

In CSS2,CSS 2.1, a box may be laid out according to three positioning schemes:

1. Normal flow. In CSS2,CSS 2.1, normal flow includes block formatting of blockblock-level boxes, inline formatting of inline boxes, relative positioning of block or inlineinline-level boxes, and relative positioning of compactblock-level and run-ininline-level boxes.
2. Floats. In the float model, a box is first laid out according to the normal flow, then taken out of the flow and shifted to the left or right as far as possible. Content may flow along the side of a float.
3. Absolute positioning. In the absolute positioning model, a box is removed from the normal flow entirely (it has no impact on later siblings) and assigned a position with respect to a containing block.

An element is called out of flow if it is floated, absolutely positioned, or is the root element. An element is called in-flow if it is not out-of-flow. The flow of an element A is the set consisting of A and all in-flow elements whose nearest out-of-flow ancestor is A.

9.3.1 Choosing a positioning scheme: 'position' property

The 'position' and 'float' properties determine which of the CSS2CSS 2.1 positioning algorithms is used to calculate the position of a box.

The values of this property have the following meanings:

static

The box is a normal box, laid out according to the normal flow. The 'left' and'top', 'right', 'bottom', and 'left' properties do not apply.

relative

The box's position is calculated according to the normal flow (this is called the position in normal flow). Then the box is offset relative to its normal position. When a box B is relatively positioned, the position of the following box is calculated as though B were not offset. The effect of 'position:relative' on table-row-group, table-header-group, table-footer-group, table-row, table-column-group, table-column, table-cell, and table-caption elements is undefined.

absolute

The box's position (and possibly size) is specified with the 'left''top', 'right', 'top''bottom', and 'bottom''left' properties. These properties specify offsets with respect to the box's containing block. Absolutely positioned boxes are taken out of the normal flow. This means they have no impact on the layout of later siblings. Also, though absolutely positioned boxes have margins, they do not collapse with any other margins.

fixed

The box's position is calculated according to the 'absolute' model, but in addition, the box is fixed with respect to some reference. As with the 'absolute' model, the box's margins do not collapse with any other margins. In the case of continuoushandheld, projection, screen, tty, and tv media ,types, the box is fixed with respect to the viewport (and doesn'tand does not move when scrolled).scrolled. In the case of pagedthe print media ,type, the box is rendered on every page, and is fixed with respect to the page,page box, even if thatthe page is seen through a viewport (in the case of a print-preview, for example). For other media types, the presentation is undefined. Authors may wish to specify 'fixed' in a media-dependent way. For instance, an author may want a box to remain at the top of the viewport on the screen, but not at the top of each printed page. The two specifications may be separated by using an @media rule, as in:

Example(s):

   
@media screen { 
  h1#first { position: fixed } 
}
@media print { 
  h1#first { position: static }
}

9.3.2 Box offsets : 'top'UAs must not paginate the content of fixed boxes. Note that UAs may print invisible content in other ways. See "Content outside the page box" in chapter 13.

User agents may treat position as 'static' on the root element.

9.3.2 Box offsets: 'top', 'right', 'bottom', 'left'

An element is said to be positioned if its 'position' property has a value other than 'static'. Positioned elements generate positioned boxes, laid out according to four properties:

This property specifies how far aan absolutely positioned box's top contentmargin edge is offset below the top edge of the box's containing block. For relatively positioned boxes, the offset is with respect to the top edges of the box itself (i.e., the box is given a position in the normal flow, then offset from that position according to these properties).

Like 'top', but specifies how far a box's right contentmargin edge is offset to the left of the right edge of the box's containing block. For relatively positioned boxes, the offset is with respect to the right edge of the box itself.

Like 'top', but specifies how far a box's bottom contentmargin edge is offset above the bottom of the box's containing block. For relatively positioned boxes, the offset is with respect to the bottom edge of the box itself.

Like 'top', but specifies how far a box's left contentmargin edge is offset to the right of the left edge of the box's containing block. For relatively positioned boxes, the offset is with respect to the left edge of the box itself.

The values for the four properties have the following meanings:

<length>

The offset is a fixed distance from the reference edge. Negative values are allowed.

<percentage>

The offset is a percentage of the containing block's width (for 'left' or 'right') or height (for 'top' and 'bottom'). For 'top' and 'bottom', if the height of the containing block is not specified explicitly (i.e., it depends on content height), the percentage value is interpreted like 'auto'.Negative values are allowed.

auto

For non-replaced elements, the effect of this value depends on which of related properties have the value 'auto' as well. See the sections on the width and height of absolutely positioned, non-replaced elements for details. For absolutely positioned boxes, the offsets are with respect toreplaced elements, the box's containing block . For relatively positioned boxes, the offsets are with respect toeffect of this value depends only on the outer edgesintrinsic dimensions of the box itself (i.e.,replaced content. See the box is given a position insections on the normal flow, then offset from that position according to these properties).width and height of absolutely positioned, replaced elements for details.

9.4 Normal flow

Boxes in the normal flow belong to a formatting context, which may be block or inline, but not both simultaneously. BlockBlock-level boxes participate in a block formatting context. InlineInline-level boxes participate in an inline formatting context.

9.4.1 Block formatting contextcontexts

Floats, absolutely positioned elements, block containers (such as inline-blocks, table-cells, and table-captions) that are not block boxes, and block boxes with 'overflow' other than 'visible' (except when that value has been propagated to the viewport) establish new block formatting contexts for their contents.

In a block formatting context, boxes are laid out one after the other, vertically, beginning at the top of a containing block. The vertical distance between two sibling boxes is determined by the 'margin' properties. Vertical margins between adjacent blockblock-level boxes in a block formatting context collapse.

In a block formatting context, each box's left outer edge touches the left edge of the containing block (for right-to-left formatting, right edges touch). This is true even in the presence of floats (although a box's content arealine boxes may shrink due to the floats), unless the box establishes a new block formatting context (in which case the box itself may become narrower due to the floats).

For information about page breaks in paged media, please consult the section on allowed page breaks.

9.4.2 Inline formatting contextcontexts

In an inline formatting context, boxes are laid out horizontally, one after the other, beginning at the top of a containing block. Horizontal margins, borders, and padding are respected between these boxes. The boxes may be aligned vertically in different ways: their bottoms or tops may be aligned, or the baselines of text within them may be aligned. The rectangular area that contains the boxes that form a line is called a line box.

The width of a line box is determined by a containing block .and the presence of floats. The height of a line box is determined by the rules given in the section on line height calculations.

A line box is always tall enough for all of the boxes it contains. However, it may be taller than the tallest box it contains (if, for example, boxes are aligned so that baselines line up). When the height of a box B is less than the height of the line box containing it, the vertical alignment of B within the line box is determined by the 'vertical-align' property. When several inlineinline-level boxes cannot fit horizontally within a single line box, they are distributed among two or more vertically-stacked line boxes. Thus, a paragraph is a vertical stack of line boxes. Line boxes are stacked with no vertical separation (except as specified elsewhere) and they never overlap.

In general, the left edge of a line box touches the left edge of its containing block and the right edge touches the right edge of its containing block. However, floating boxes may come between the containing block edge and the line box edge. Thus, although line boxes in the same inline formatting context generally have the same width (that of the containing block), they may vary in width if available horizontal space is reduced due to floats. Line boxes in the same inline formatting context generally vary in height (e.g., one line might contain a tall image while the others contain only text).

When the total width of the inlineinline-level boxes on a line is less than the width of the line box containing them, their horizontal distribution within the line box is determined by the 'text-align' property. If that property has the value 'justify', the user agent may stretch thespaces and words in inline boxes (but not inline-table and inline-block boxes) as well.

SinceWhen an inline box may not exceedexceeds the width of a line box, long inline boxes areit is split into several boxes and these boxes are distributed across several line boxes. If an inline box cannot be split (e.g., if the inline box contains a single character, or language specific word breaking rules disallow a break within the inline box, or if the inline box is affected by a white-space value of nowrap or pre), then the inline box overflows the line box.

When an inline box is split, margins, borders, and padding have no visual effect where the split occurs. Formatting of margins, borders, and padding may not be fully defined if the splitoccurs within a bidirectional embedding.(or at any split, when there are several).

Inline boxes may also be split into several boxes within the same line box due to bidirectional text processing.

Here isLine boxes are created as needed to hold inline-level content within an example ofinline box construction. The following paragraph (created by the HTML block-level element P) contains anonymous text interspersed with the elements EM and STRONG: <P>Several <EM>emphasized words</EM>formatting context. Line boxes that contain no text, no preserved white space, no inline elements with non-zero margins, padding, or borders, and no other in-flow content (such as images, inline blocks or inline tables), and do not end with a preserved newline must be treated as zero-height line boxes for the purposes of determining the positions of any elements inside of them, and must be treated as not existing for any other purpose.

<P>Several <EM>emphasized words</EM> appear
<STRONG>in this</STRONG> sentence, dear.</P>

 Several emphasized words appear in this sentence, dear.

Several emphasized words appear
in this sentence, dear.

Several emphasized  
words appear in this 
sentence, dear.

In the previous example, the EM box was split into two EM boxes (call them "split1" and "split2"). Margins, borders, padding, or text decorations have no visible effect after split1 or before split2.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML  4.0//EN">4.01//EN">
<HTML>
  <HEAD>
    <TITLE>Example of inline flow on several lines</TITLE>
    <STYLE type="text/css">
      EM {
        padding: 2px; 
        margin: 1em;
        border-width: medium;
        border-style: dashed;
        line-height: 2.4em;
      }
    </STYLE>
  </HEAD>
  <BODY>
    <P>Several <EM>emphasized words</EM> appear here.</P>
  </BODY>
</HTML>

9.4.3 Relative positioning

Once a box has been laid out according to the normal flow ,or floated, it may be shifted relative to this position. This is called relative positioning. Offsetting a box (B1) in this way has no effect on the box (B2) that follows: B2 is given a position as if B1 were not offset and B2 is not re-positioned after B1's offset is applied. This implies that relative positioning may cause boxes to overlap. However, if relative positioning causes an 'overflow:auto' or 'overflow:scroll' box to have overflow, the UA must allow the user to access this content (at its offset position), which, through the creation of scrollbars, may affect layout.

A relatively positioned boxes keep theirbox keeps its normal flow size, including line breaks and the space originally reserved for them.it. The section on containing blocks explains when a relatively positioned box establishes a new a newcontaining blockblock.

For normal flow children and positioned descendants. Arelatively positioned box is generated when the 'position' property for an element haselements, 'left' and 'right' move the value 'relative'.box(es) horizontally, without changing their size. 'Left' moves the offset is specified byboxes to the 'top' , 'bottom' , 'left' ,right, and 'right' properties. Dynamic movement of relatively positioned boxes can produce animation effects in scripting environments (see alsomoves them to the 'visibility' property). Relative positioning may also be usedleft. Since boxes are not split or stretched as a general formresult of superscripting'left' or 'right', the used values are always: left = -right.

If both 'left' and subscripting except that line height'right' are 'auto' (their initial values), the used values are '0' (i.e., the boxes stay in their original position).

If 'left' is not automatically adjusted to take'auto', its used value is minus the value of 'right' (i.e., the boxes move to the left by the value of 'right').

If 'right' is specified as 'auto', its used value is minus the value of 'left'.

If neither 'left' nor 'right' is 'auto', the position is over-constrained, and one of them has to be ignored. If the 'direction' property of the containing block is 'ltr', the value of 'left' wins and 'right' becomes -'left'. If 'direction' of the containing block is 'rtl', 'right' wins and 'left' is ignored.

div.a8 { position: relative; direction: ltr; left: -1em; right: auto }
div.a8 { position: relative; direction: ltr; left: auto; right: 1em }
div.a8 { position: relative; direction: ltr; left: -1em; right: 5em }

The 'top' and 'bottom' properties move relatively positioned element(s) up or down without changing their size. 'Top' moves the boxes down, and 'bottom' moves them up. Since boxes are not split or stretched as a result of 'top' or 'bottom', the used values are always: top = -bottom. If both are 'auto', their used values are both '0'. If one of them is 'auto', it becomes the negative of the other. If neither is 'auto', 'bottom' is ignored (i.e., the used value of 'bottom' will be minus the value of 'top').

Note. Dynamic movement of relatively positioned boxes can produce animation effects in scripting environments (see also the 'visibility' property). Although relative positioning may be used as a form of superscripting and subscripting, the line height is not automatically adjusted to take the positioning into consideration. See the description of line height calculations for more information.

Examples of relative positioning are provided in the section comparing normal flow, floats, and absolute positioning.

9.5 Floats

A float is a box that is shifted to the left or right on the current line. The most interesting characteristic of a float (or "floated" or "floating" box) is that content may flow along its side (or be prohibited from doing so by the 'clear' property). Content flows down the right side of a left-floated box and down the left side of a right-floated box. The following is an introduction to float positioning and content flow; the exact rules governing float behavior are given in the description of the 'float' property.

A floated box is shifted to the left or right until its outer edge touches the containing block edge or the outer edge of another float. If there is a line box, the outer top of the floated box is aligned with the top of the current line box.

If there is not enough horizontal room for the float, it is shifted downward until either it fits or there are no more floats present.

Since a float is not in the flow, non-positioned block boxes created before and after the float box flow vertically as if the float did not exist. However, the current and subsequent line boxes created next to the float are shortened as necessary to make room for the margin box of the float.

A line box is next to a float when there exists a vertical position that satisfies all of these four conditions: (a) at or below the top of the line box, (b) at or above the bottom of the line box, (c) below the top margin edge of the float, and (d) above the bottom margin edge of the float.

Note: this means that floats with zero outer height or negative outer height do not shorten line boxes.

If a shortened line box is too small to contain any content, then the line box is shifted downward (and its width recomputed) until either some content fits or there are no more floats present. Any content in the current line before a floated box is reflowed in the same line on the other side of the float. In other words, if inline-level boxes are placed on the line before a left float is encountered that fits in the remaining line box space, the left float is placed on that line, aligned with the top of the line box, and then the inline-level boxes already on the line are moved accordingly to the right of the float (the right being the other side of the left float) and vice versa for rtl and right floats.

The border box of a table, a block-level replaced element, or an element in the normal flow that establishes a new block formatting context (such as an element with 'overflow' other than 'visible') must not overlap the margin box of any floats in the same block formatting context as the element itself. If necessary, implementations should clear the said element by placing it below any preceding floats, but may place it adjacent to such floats if there is sufficient space. They may even make the border box of said element narrower than defined by section 10.3.3. CSS2 does not define when a UA may put said element next to the float or by how much said element may become narrower.

p { width: 10em; border: solid aqua; }
span { float: left; width: 5em; height: 5em; border: solid blue; }


...


<p>
  <span> </span>
  Supercalifragilisticexpialidocious
</p>

Several floats may be adjacent, and this model also applies to adjacent floats in the same line.

img.icon { 
  float: left;
  margin-left: 0;
}

  
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
  <HEAD>
    <TITLE>Float example</TITLE>
    <STYLE type="text/css">
      IMG { float: left }
      BODY, P, IMG { margin: 2em }
    </STYLE>
  </HEAD>
  <BODY>
    <P><IMG src=img.png alt="This image will illustrate floats">
       Some sample text that has no other...
  </BODY>
</HTML>

<BODY>
  <P>Some sample text 
  <IMG src=img.png alt="This image will illustrate floats">
           that has no other...
</BODY>

As stated in section 8.3.1, the margins of floating boxes never collapse with margins of adjacent boxes. Thus, in the previous example, vertical margins do not collapse between the P box and the floated IMG box.

The contents of floats are stacked as if floats generated new stacking contexts, except that any positioned elements and elements that actually create new stacking contexts take part in the float's parent stacking context. A float can overlap other boxes in the normal flow (e.g., when a normal flow box next to a float has negative margins). When this happens, floats are rendered in front of non-positioned in-flow blocks, but behind in-flow inlines.

The positioning into consideration. Seefollowing example illustrates the description of line height calculations for more information. Examplesuse of relative positioning are provided inthe section comparing normal flow, floats, and absolute positioning . 9.5 Floats'clear' property to prevent content from flowing next to a float isfloat.

p { clear: left  or right on the current line.}

Content flows down9.5.1 Positioning the right side of a left-floated box and downfloat: the 'float' property

This property specifies whether a right-floated box. The following is an introduction tobox should float positioning and content flow;to the exact rules governing float behaviorleft, right, or not at all. It may be set for any element, but only applies to elements that generate boxes that are given innot absolutely positioned. The descriptionvalues of the 'float' property. A floated box mustthis property have an explicit width (assigned viathe 'width' property, or its intrinsic width infollowing meanings:

left

The case of replaced elements ). Any floated box becomeselement generates a block box that is shiftedfloated to the left or right until its outer edge touches the containing block edge or the outer edge of another float.left. Content flows on the topright side of the floated box is aligned withbox, starting at the top of the current line box (or bottom of the preceding block box if no line box exists). If there isn't enough horizontal room on the current line for the float, it is shifted downward, line by line, until a line has room for it. Since a float is not in the flow, non-positioned block boxes created before and after the float box flow vertically as if the float didn't exist. However, line boxes created next(subject to the float are shortened'clear' property).

right

Similar to make room for the floated box. Any content in'left', except the current line before a floatedbox is reflowed infloated to the first available lineright, and content flows on the otherleft side of the float. Several floats may be adjacent, and this model also applies to adjacent floats in the same line. Example(s): The following rule floats all IMG boxes with class="icon" tobox, starting at the left (and setstop.

none

The left margin to '0'): IMG.icon { float: left; margin-left: 0; } Considerbox is not floated.

User agents may treat float as 'none' on the following HTML source and style sheet: <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN"> <HTML> <HEAD> <TITLE>Float example</TITLE> <STYLE type="text/css"> IMG { float: left } BODY, P, IMG { margin: 2em } </STYLE> </HEAD> <BODY> <P><IMG src=img.gif alt="This image will illustrate floats"> Some sample text that has no other... </BODY> </HTML>root element.

Here are the IMGprecise rules that govern the behavior of floats:

1. The left outer edge of a left-floating box is floatedmay not be to the left.left of the content that followsleft edge of its containing block. An analogous rule holds for right-floating elements.
2. If the current box is formattedleft-floating, and there are any left-floating boxes generated by elements earlier in the source document, then for each such earlier box, either the left outer edge of the current box must be to the right of the float, starting onright outer edge of the same line asearlier box, or its top must be lower than the float.bottom of the line boxesearlier box. Analogous rules hold for right-floating boxes.
3. The right outer edge of a left-floating box may not be to the right of the float are shortened dueleft outer edge of any right-floating box that is next to it. Analogous rules hold for right-floating elements.
4. A floating box's outer top may not be higher than the float's presence, but resume their "normal" width (thattop of theits containing block established by the P element) after. When the float. This document might be formatted as:     [D] Formatting would have been exactlyfloat occurs between two collapsing margins, the samefloat is positioned as if the documentit had been: <BODY> <P>Some sample text <IMG src=img.gif alt="This image will illustrate floats"> that has no other... </BODY> becausean otherwise empty anonymous block parent taking part in the content toflow. The leftposition of the floatsuch a parent is displaceddefined by the float and reflowed down its right side.rules in the section on margin collapsing.
5. The outer top of a floating box may not be higher than the margins of floating boxes never collapse with marginsouter top of adjacent boxes. Thus,any block or floated box generated by an element earlier in the previous example, vertical margins do not collapse betweensource document.
6. The Pouter top of an element's floating box andmay not be higher than the floated IMG box.top of any line-box containing a float can overlap other boxesbox generated by an element earlier in the normal flow (e.g., whensource document.
7. A normal flowleft-floating box nextthat has another left-floating box to its left may not have its right outer edge to the right of its containing block's right edge. (Loosely: a left float has negative margins). Whenmay not stick out at the right edge, unless it is already as far to the left as possible.) An inlineanalogous rule holds for right-floating elements.
8. A floating box overlaps withmust be placed as high as possible.
9. A float, the content, background, and borders of the inlineleft-floating box are rendered in front ofmust be put as far to the float. Whenleft as possible, a blockright-floating box overlaps,as far to the background and borders ofright as possible. A higher position is preferred over one that is further to the block box are rendered behindleft/right.

But in CSS 2.1, if, within the float and are only be visible whereblock formatting context, there is an in-flow negative vertical margin such that the boxfloat's position is transparent.above the contentposition it would be at were all such negative margins set to zero, the position of the block boxfloat is renderedundefined.

References to other elements in these rules refer only to other elements in front ofthe same block formatting context as the float.

<P>a<SPAN style="float: right">b</SPAN></P>

    [D] Both paragraphs have set 'clear: left', which causes the second paragraph to be "pushed down" to a position below the float -- its top margin expands9.5.2 Controlling flow next to accomplish this (seefloats: the 'clear' property). 9.5.1 Positioning the float: the 'float'property

This property specifies whether a box should float to the left, right, or not at all. Itindicates which sides of an element's box(es) may be set for elements that generate boxes that arenot absolutely positioned . The values of this property have the following meanings: left The element generates a block box that is floated to the left. Content flows on the right side of the box, starting at the top (subjectbe adjacent to an earlier floating box. The 'clear' property). The 'display' is ignored, unless it has the value 'none'. right Same as 'left', but content flows on the left side of the box, starting at the top. none The box isproperty does not floated. Here are the precise rules that governconsider floats inside the behavior of floats:element itself or in other block formatting contexts.

Values have the following meanings when applied to non-floating block-level boxes:

left

outerRequires that the top border edge of a left-floatingthe box may notbe to the left ofbelow the leftbottom outer edge of its containing block . An analogous rule holds for right-floating elements. If the current box is left-floating, and there areany left floatingleft-floating boxes generated bythat resulted from elements earlier in the source document, then for each such earlier box, eitherdocument.

right

Requires that the left outertop border edge of the currentbox mustbe to the right ofbelow the rightbottom outer edge of theany right-floating boxes that resulted from elements earlier box, or its top must be lower than the bottom ofin the earlier box. Analogous rules hold for right-floating boxes.source document.

both

Requires that the right outertop border edge of a left-floatingthe box may notbe to the right ofbelow the leftbottom outer edge of any right-floating boxand left-floating boxes that resulted from elements earlier in the source document.

none

No constraint on the box's position with respect to floats.

Values other than 'none' potentially introduce clearance. Clearance inhibits margin collapsing and acts as spacing above the margin-top of an element. It is used to push the rightelement vertically past the float.

Computing the clearance of it. Analogous rules hold for right-floating elements. A floating box's outeran element on which 'clear' is set is done by first determining the hypothetical position of the element's top mayborder edge. This position is where the actual top border edge would have been if the element's 'clear' property had been 'none'.

If this hypothetical position of the element's top border edge is not be higher thanpast the relevant floats, then clearance is introduced, and margins collapse according to the top of its containing block .rules in 8.3.1.

Then the outer topamount of a floating box may not be higher thanclearance is set to the outer top of any block or floated box generated by an element earlier ingreater of:

1. The source document.amount necessary to place the outer topborder edge of an element's floating box may not be higher thanthe topblock even with the bottom outer edge of any line-box containing a box generated by an element earlier inthe source document. A left-floating boxlowest float that has another left-floating boxis to its left may not have its right outer edgebe cleared.
2. The amount necessary to place the righttop border edge of its containing block's right edge. (Loosely: a left float may not stick out atthe right edge, unless itblock at its hypothetical position.

Alternatively, clearance is already as farset exactly to the left as possible.) An analogous rule holds for right-floating elements. A floating box must be placed as high as possible. A left-floating box must be put as faramount necessary to place the left as possible, a right-floating box as far toborder edge of the right as possible. A higher position is preferred over oneblock even with the bottom outer edge of the lowest float that is further to the left/right. 9.5.2 Controlling flow nextto floats: the 'clear' property 'clear' Value:    none | left | right |be cleared.

Note: Both | inherit Initial:    none Applies to:    block-level elements Inherited:    no Percentages:    N/A Media:    visual This property indicates which sidesbehaviors are allowed pending evaluation of an element's box(es) may not be adjacent to an earlier floating box. (It maytheir compatibility with existing Web content. A future CSS specification will require either one or the other.

Note: The clearance can be thatnegative or zero.

<p style="margin-bottom: 4em">
  First paragraph.

<p style="float: left; height: 2em; margin: 0">
  Floating paragraph.

<p style="clear: left; margin-top: 3em">
  Last paragraph.

When the property is set on floating elements, it results in a modification of the rules for positioning the float. An extra constraint (#10) is added:

• The top outer edge of the float must be below the bottom outer edge of all earlier left-floating boxes (in the case of 'clear: left'), or all earlier right-floating boxes (in the case of 'clear: right'), or both ('clear: both').

9.6 Absolute positioning

In the absolute positioning model, a box is explicitly offset with respect to its containing block. It is removed from the normal flow entirely (it has no impact on later siblings). An absolutely positioned box establishes a new containing block for normal flow children and absolutely (but not fixed) positioned descendants. However, the contents of an absolutely positioned element do not flow around any other boxes. They may or may notobscure the contents of another box,box (or be obscured themselves), depending on the stack levels of the overlapping boxes.

References in this specification to an absolutely positioned element (or its box) imply that the element's 'position' property has the value 'absolute' or 'fixed'.

9.6.1 Fixed positioning

Fixed positioning is a subcategory of absolute positioning. The only difference is that for a fixed positioned box, the containing block is established by the viewport. For continuous media, fixed boxes do not move when the document is scrolled. In this respect, they are similar to fixed background images. For paged media, boxes with fixed positions are repeated on every page. This is useful for placing, for instance, a signature at the bottom of each page. Boxes with fixed position that are larger than the page area are clipped. Parts of the fixed position box that are not visible in the initial containing block will not print.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML  4.0//EN">4.01//EN">
<HTML>
  <HEAD>
    <TITLE>A frame document with  CSS2</TITLE>CSS 2.1</TITLE>
    <STYLE  type="text/css">type="text/css" media="screen">
      BODY { height: 8.5in } /* Required for percentage heights below */
      #header {
        position: fixed;
        width: 100%;
        height: 15%;
        top: 0;
        right: 0;
        bottom: auto;
        left: 0;
      }
      #sidebar {
        position: fixed;
        width: 10em;
        height: auto;
        top: 15%;
        right: auto;
        bottom: 100px;
        left: 0;
      }
      #main {
        position: fixed;
        width: auto;
        height: auto;
        top: 15%;
        right: 0;
        bottom: 100px;
        left: 10em;
      }
      #footer {
        position: fixed;
        width: 100%;
        height: 100px;
        top: auto;
        right: 0;
        bottom: 0;
        left: 0;
      }
    </STYLE>
  </HEAD>
  <BODY>
    <DIV id="header"> ...  </DIV>
    <DIV id="sidebar"> ...  </DIV>
    <DIV id="main"> ...  </DIV>
    <DIV id="footer"> ...  </DIV>
  </BODY>
</HTML>

9.7 Relationships between 'display', 'position', and 'float'

The three properties that affect box generation and layout --— 'display', 'position', and 'float' --— interact as follows:

1. If 'display' has the value 'none', user agents must ignorethen 'position' and 'float' .do not apply. In this case, the element generates no box.
2. Otherwise, if 'position' has the value 'absolute' or 'fixed', 'display'the box is set to 'block' andabsolutely positioned, the computed value of 'float' is 'none', and display is set according to 'none'.the table below. The position of the box will be determined by the 'top', 'right', 'bottom' and 'left' properties and the box's containing block.
3. Otherwise, if 'float' has a value other than 'none', the box is floated and 'display' is set according to the table below.
4. Otherwise, if the element is the root element, 'display' is set according to 'block' andthe boxtable below, except that it is floated.undefined in CSS 2.1 whether a specified value of 'list-item' becomes a computed value of 'block' or 'list-item'.
5. Otherwise, the remaining 'display' propertiesproperty values apply as specified.

9.8 Comparison of normal flow, floats, and absolute positioning

To illustrate the differences between normal flow, relative positioning, floats, and absolute positioning, we provide a series of examples based on the following HTML fragment:HTML:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML  4.0//EN">4.01//EN">
<HTML>
  <HEAD>
    <TITLE>Comparison of positioning schemes</TITLE>
  </HEAD>
  <BODY>
    <P>Beginning of body contents.
      <SPAN id="outer"> Start of outer contents.
      <SPAN id="inner"> Inner contents.</SPAN>
      End of outer contents.</SPAN>
      End of body contents.
    </P>
  </BODY>
</HTML>

In this document, we assume the following rules:

body { display: block; font-size:12px; line-height: 200%; 
       width: 400px; height: 400px }
p    { display: block }
span { display: inline }

The final positions of boxes generated by the outer and inner elements vary in each example. In each illustration, the numbers to the left of the illustration indicate the normal flow position of the double-spaced (for clarity) lines.

(Note:Note. The illustrations use different horizontaldiagrams in this section are illustrative and vertical scales.)not to scale. They are meant to highlight the differences between the various positioning schemes in CSS 2.1, and are not intended to be reference renderings of the examples given.

9.8.1 Normal flow

Consider the following CSS declarations for outer and inner that don'tdo not alter the normal flow of boxes:

#outer { color: red }
#inner { color: blue }

The P element contains all inline content: anonymous inline text and two SPAN element.elements. Therefore, all of the content will be laid out in an inline formatting context, within a containing block established by the P element, producing something like:

9.8.2 Relative positioning

To see the effect of relative positioning, we specify:

#outer { position: relative; top: -12px; color: red }
#inner { position: relative; top: 12px; color: blue }

Text flows normally up to the outer element. The outer text is then flowed into its normal flow position and dimensions at the end of line 1. Then, the inline boxes containing the text (distributed over three lines) are shifted as a unit by '-12px' (upwards).

The contents of inner, as a child of outer, would normally flow immediately after the words "of outer contents" (on line 1.5). However, the inner contents are themselves offset relative to the outer contents by '12px' (downwards), back to their original position on line 2.

Note that the content following outer is not affected by the relative positioning of outer.

Note also that had the offset of outer been '-24px', the text of outer and the body text would have overlapped.

9.8.3 Floating a box

Now consider the effect of floating the inner element's text to the right by means of the following rules:

#outer { color: red }
#inner { float: right; width: 130px; color: blue }

Text flows normally up to the inner box, which is pulled out of the flow and floated to the right margin (its 'width' has been assigned explicitly). Line boxes to the left of the float are shortened, and the document's remaining text flows into them.

To show the effect of the 'clear' property, we add a sibling element to the example:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML  4.0//EN">4.01//EN">
<HTML>
  <HEAD>
    <TITLE>Comparison of positioning schemes II</TITLE>
  </HEAD>
  <BODY>
    <P>Beginning of body contents.
      <SPAN id=outer> Start of outer contents.
      <SPAN id=inner> Inner contents.</SPAN>
      <SPAN id=sibling> Sibling contents.</SPAN>
      End of outer contents.</SPAN>
      End of body contents.
    </P>
  </BODY>
</HTML>

The following rules:

#inner { float: right; width: 130px; color: blue }
#sibling { color: red }

cause the inner box to float to the right as before and the document's remaining text to flow into the vacated space:

However, if the 'clear' property on the sibling element is set to 'right' (i.e., the generated sibling box will not accept a position next to floating boxes to its right), the sibling content begins to flow below the float:

#inner { float: right; width: 130px; color: blue }
#sibling { clear: right; color: red }

9.8.4 Absolute positioning

Finally, we consider the effect of absolute positioning. Consider the following CSS declarations for outer and inner:

#outer { 
    position: absolute; 
    top: 200px; left: 200px; 
    width: 200px; 
    color: red;
}
#inner { color: blue }

which cause the top of the outer box to be positioned with respect to its containing block. The containing block for a positioned box is established by the nearest positioned ancestor (or, if none exists, the initial containing block, as in our example). The top side of the outer box is '200px' below the top of the containing block and the left side is '200px' from the left side. The child box of outer is flowed normally with respect to its parent.

The following example shows an absolutely positioned box that is a child of a relatively positioned box. Although the parent outer box is not actually offset, setting its 'position' property to 'relative' means that its box may serve as the containing block for positioned descendants. Since the outer box is an inline box that is split across several lines, the first inline box's top and left edges (depicted by thick dashed lines in the illustration below) serve as references for 'top' and 'left' offsets.

#outer { 
  position: relative; 
  color: red 
}
#inner { 
  position: absolute; 
  top: 200px; left: -100px; 
  height: 130px; width: 130px; 
  color: blue;
}

This results in something like the following:

If we do not position the outer box:

#outer { color: red }
#inner {
  position: absolute; 
  top: 200px; left: -100px; 
  height: 130px; width: 130px; 
  color: blue;
}

the containing block for inner becomes the initial containing block (in our example). The following illustration shows where the inner box would end up in this case.

<P style="position: relative; margin-right: 10px; left: 10px;">
I used two red hyphens to serve as a change bar. They
will "float" to the left of the line containing THIS
<SPAN style="position: absolute; top: auto; left: -1em; color: red;">--</SPAN>
word.</P>

9.9 Layered presentation

9.9.1 Specifying the stack level: the 'z-index' property

For a positioned box, the 'z-index' property specifies:

1. The stack level of the box in the current stacking context.
2. Whether the box establishes a stacking context.

Values have the following meanings:

<integer>

This integer is the stack level of the generated box in the current stacking context. The box also establishes a new stacking context.

auto

The stack level of the Pgenerated box in its final position).the resultcurrent stacking context is that the change bars seem to "float" to0. The left ofbox does not establish a new stacking context unless it is the current line. 9.9 Layered presentationroot element.

In the following sections,this section, the expression "in front of" means closer to the user as the user faces the screen.

In CSS2,CSS 2.1, each box has a position in three dimensions. In addition to their horizontal and vertical positions, boxes lie along a "z-axis" and are formatted one on top of the other. Z-axis positions are particularly relevant when boxes overlap visually. This section discusses how boxes may be positioned along the z-axis.

Each box belongs to one stacking context . Each boxThe order in a given stacking context has an integer stack level ,which is its position onthe z-axis relative to other boxes inrendering tree is painted onto the same stacking context. Boxes with greater stack levels are always formattedcanvas is described in frontterms of boxes with lower stack levels. Boxes may have negative stack levels. Boxes with the same stack level in a stacking context are stacked bottom-to-top according to document tree order. The root element creates a rootstacking context , but other elements may establish localcontexts. Stacking contexts .can contain further stacking contexts are inherited.contexts. A localstacking context is atomic;atomic from the point of view of its parent stacking context; boxes in other stacking contexts may not come between any of its boxes.

An element that establishes a local stacking context generates aEach box that has two stack levels:belongs to one for thestacking context it creates (always '0') and one for the. Each positioned box in a given stacking context to which it belongs (given by the 'z-index' property). An element's boxhas the samean integer stack level as, which is its parent's box unless given a different stack level with the 'z-index' property. 9.9.1 Specifying the stack level : the 'z-index' property 'z-index' Value:    auto | <integer> | inherit Initial:    auto Applies to:    positioned elements Inherited:    no Percentages:    N/A Media:    visual For a positioned box, the 'z-index' property specifies:position on the z-axis relative other stack levellevels within the same stacking context. Boxes with greater stack levels are always formatted in front of boxes with lower stack levels. Boxes may have negative stack levels. Boxes with the boxsame stack level in a stacking context are stacked back-to-front according to document tree order.

The currentroot element forms the root stacking context. Whether the box establishesOther stacking contexts are generated by any positioned element (including relatively positioned elements) having a localcomputed value of 'z-index' other than 'auto'. Stacking context. Values havecontexts are not necessarily related to containing blocks. In future levels of CSS, other properties may introduce stacking contexts, for example 'opacity' [CSS3COLOR].

Within each stacking context, the following meanings: <integer> This integer islayers are painted in back-to-front order:

1. the stack levelbackground and borders of the generated box inelement forming the currentstacking context.
2. the box also establishes a localchild stacking context in which itscontexts with negative stack levels (most negative first).
3. the in-flow, non-inline-level, non-positioned descendants.
4. the non-positioned floats.
5. the in-flow, inline-level, non-positioned descendants, including inline tables and inline blocks.
6. the child stacking contexts with stack level is '0'. auto0 and the positioned descendants with stack level of0.
7. the child stacking contexts with positive stack levels (least positive first).

Within each stacking context, positioned elements with stack level 0 (in layer 6), non-positioned floats (layer 4), inline blocks (layer 5), and inline tables (layer 5), are painted as if those elements themselves generated boxnew stacking contexts, except that their positioned descendants and any would-be child stacking contexts take part in the current stacking contextcontext.

This painting order is the same as its parent's box. The box does not establish a new localapplied recursively to each stacking context. This description of stacking context painting order constitutes an overview of the detailed normative definition in Appendix E.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML  4.0//EN">4.01//EN">
<HTML>
  <HEAD>
    <TITLE>Z-order positioning</TITLE>
    <STYLE type="text/css">
      .pile { 
        position: absolute; 
        left: 2in; 
        top: 2in; 
        width: 3in; 
        height: 3in; 
      }
    </STYLE>
  </HEAD>
  <BODY>
    <P>
      <IMG id="image" class="pile" 
            src="butterfly.gif"src="butterfly.png" alt="A butterfly image"
           style="z-index: 1">

    <DIV id="text1" class="pile" 
         style="z-index: 3">
      This text will overlay the butterfly image.
    </DIV>

    <DIV id="text2">
      This text will be beneath everything.
    </DIV>

    <DIV id="text3" class="pile" 
         style="z-index: 2">
      This text will underlay text1, but overlay the butterfly image
    </DIV>
  </BODY>
</HTML>

This example demonstrates the notion of transparency. The default behavior of a boxthe background is to allow boxes behind it to be visible through transparent areas in its content.visible. In the example, each box transparently overlays the boxes below it. This behavior can be overridden by using one of the existing background properties.

9.10 Text direction: the 'direction' and 'unicode-bidi' properties

Conforming user agents that do not support bidirectional text may ignore the 'direction' and 'unicode-bidi' properties described in this section. This exception includes UAs that render right-to-left characters simply because a font on the system contains them but do not support the concept of right-to-left text direction.

The characters in certain scripts are written from right to left. In some documents, in particular those written with the Arabic or Hebrew script, and in some mixed-language contexts, text in a single (visually displayed) block may appear with mixed directionality. This phenomenon is called bidirectionality, or "bidi" for short.

The Unicode standard ([UNICODE], section 3.11)[UAX9]) defines a complex algorithm for determining the proper directionality of text. The algorithm consists of an implicit part based on character properties, as well as explicit controls for embeddings and overrides. CSS2CSS 2.1 relies on this algorithm to achieve proper bidirectional rendering. The 'direction' and 'unicode-bidi' properties allow authors to specify how the elements and attributes of a document language map to this algorithm.

If a document contains right-to-left characters, and if the user agent displays these characters (with appropriate glyphs, not arbitrary substitutes such as a question mark, a hex code, a black box, etc.), theUser agentagents that support bidirectional text must apply the Unicode bidirectional algorithm. This seemingly one-sided requirement reflects the fact that, although notalgorithm to every Hebrewsequence of inline-level boxes uninterrupted by a forced (bidi class B) break or Arabic document contains mixed-directionality text, such documents are much more likelyblock boundary. This sequence forms the "paragraph" unit in the bidirectional algorithm. The paragraph embedding level is set according to contain left-to-right text (e.g., numbers, text from other languages)the value of the 'direction' property of the containing block rather than are documents writtenby the heuristic given in left-to-right languages.steps P2 and P3 of the Unicode algorithm.

Because the directionality of a text depends on the structure and semantics of the document language, these properties should in most cases be used only by designers of document type descriptions (DTDs), or authors of special documents. If a default style sheet specifies these properties, authors and users should not specify rules to override them.

A typical exception would be to override bidi behavior in a user agent if that user agent transliterates Yiddish (usually written with Hebrew letters) to Latin letters at the user's request.The HTML 4.04 specification ([HTML40],([HTML4], section 8.2) defines bidirectionality behavior for HTML elements. Conforming HTML user agents may therefore ignore the 'direction' and 'unicode-bidi' properties in author and user style sheets.The style sheet rules that would achieve the bidi behavior specified in [HTML40][HTML4] are given in the sample style sheet. The HTML 4.04 specification also contains more information on bidirectionality issues.

This property specifies the base writing direction of blocks and the direction of embeddings and overrides (see 'unicode-bidi') for the Unicode bidirectional algorithm. In addition, it specifies such things as the direction of table column layout, the direction of horizontal overflow, andthe position of an incomplete last line in a block in case of 'text-align: justify'.

Values for this property have the following meanings:

ltr

Left-to-right direction.

rtl

Right-to-left direction.

For the 'direction' property to have any effect on inline-levelaffect reordering in inline elements, the 'unicode-bidi' property's value must be 'embed' or 'override'.

Values for this property have the following meanings:

normal

The element does not open an additional level of embedding with respect to the bidirectional algorithm. For inline-levelinline elements, implicit reordering works across element boundaries.

embed

If the element is inline-level,inline, this value opens an additional level of embedding with respect to the bidirectional algorithm. The direction of this embedding level is given by the 'direction' property. Inside the element, reordering is done implicitly. This corresponds to adding a LRE (U+202A; for 'direction: ltr') or RLE (U+202B; for 'direction: rtl') at the start of the element and a PDF (U+202C) at the end of the element.

bidi-override

If the element is inline-level or a block-level element that contains only inline-level elements,For inline elements this creates an override. For block container elements this creates an override for inline-level descendants not within another block container element. This means that inside the element, reordering is strictly in sequence according to the 'direction' property; the implicit part of the bidirectional algorithm is ignored. This corresponds to adding a LRO (U+202D; for 'direction: ltr') or RLO (U+202E; for 'direction: rtl') at the start of the element or at the start of each anonymous child block box, if any, and a PDF (U+202C) at the end of the element.

The final order of characters in each block-level elementblock container is the same as if the bidi control codes had been added as described above, markup had been stripped, and the resulting character sequence had been passed to an implementation of the Unicode bidirectional algorithm for plain text that produced the same line-breaks as the styled text. In this process, non-textual entities such as imagesreplaced elements with 'display: inline' are treated as neutral characters, unless their 'unicode-bidi' property has a value other than 'normal', in which case they are treated as strong characters in the 'direction' specified for the element. All other atomic inline-level boxes are treated as neutral characters always.

Please note that in order to be able to flow inline boxes in a uniform direction (either entirely left-to-right or entirely right-to-left), more inline boxes (including anonymous inline boxes) may have to be created, and some inline boxes may have to be split up and reordered before flowing.

Because the Unicode algorithm has a limit of 1561 levels of embedding, care should be taken not to use 'unicode-bidi' with a value other than 'normal' unless appropriate. In particular, a value of 'inherit' should be used with extreme caution. However, for elements that are, in general, intended to be displayed as blocks, a setting of 'unicode-bidi: embed' is preferred to keep the element together in case display is changed to inline (see example below).

The following example shows an XML document with bidirectional text. It illustrates an important design principle: DTD designers should take bidi into account both in the language proper (elements and attributes) and in any accompanying style sheets. The style sheets should be designed so that bidi rules are separate from other style rules. The bidi rules should not be overridden by other style sheets so that the document language's or DTD's bidi behavior is preserved.

<HEBREW>
  <PAR>HEBREW1 HEBREW2 english3 HEBREW4 HEBREW5</PAR>
  <PAR>HEBREW6 <EMPH>HEBREW7</EMPH> HEBREW8</PAR>
</HEBREW>
<ENGLISH>
  <PAR>english9 english10 english11 HEBREW12 HEBREW13</PAR>
  <PAR>english14 english15 english16</PAR>
  <PAR>english17 <HE-QUO>HEBREW18 english19 HEBREW20</HE-QUO></PAR>
</ENGLISH>

/* Rules for bidi */
HEBREW, HE-QUO  {direction: rtl; unicode-bidi: embed}
ENGLISH         {direction: ltr; unicode-bidi: embed} 

/* Rules for presentation */
HEBREW, ENGLISH, PAR  {display: block}
EMPH                  {font-weight: bold}

               5WERBEH 4WERBEH english3 2WERBEH 1WERBEH

                                8WERBEH 7WERBEH 6WERBEH

english9 english10 english11 13WERBEH 12WERBEH

english14 english15 english16

english17 20WERBEH english19 18WERBEH

       2WERBEH 1WERBEH
  -EH 4WERBEH english3
                 5WERB

   -EH 7WERBEH 6WERBEH
                 8WERB

english9 english10 en-
glish11 12WERBEH
13WERBEH

english14 english15
english16

english17 18WERBEH
20WERBEH english19