CSS Paged Media Module Level 3

Shortname: css-page
Level: 3
Status: ED
Group: csswg
ED: http://dev.w3.org/csswg/css-page/
TR: http://www.w3.org/TR/css3-page/
Previous Version: http://www.w3.org/TR/2013/WD-css3-page-20130314/
!Issue Tracking: http://www.w3.org/Style/CSS/Tracker/products/13
Editor: Elika J. Etemad, Invited Expert, formerly Mozilla, http://fantasai.inkedblade.net/contact
Editor: Simon Sapin, Mozilla, formerly Kozea, http://exyr.org/about/
Ignored terms: fit, fit-position
Link defaults: css-break (property) break-after, css21 (property) display, css21 (property) min-height, css21 (property) max-height, css21 (property) min-width, css21 (property) max-width, css21 (property) margin, css-content-3 (property) counter-increment, css-content-3 (property) counter-reset
Abstract: This CSS module specifies how pages are generated and laid out to hold fragmented content in a paged presentation. It adds functionality for controlling page margins, page size and orientation, and headers and footers, and extends generated content to enable page numbering and running headers / footers. The process of paginating a flow into such generated pages is covered in [[!CSS3-BREAK]].

Introduction

Paged media (e.g., paper, transparencies, photo album pages, pages displayed on computer screens as printed output simulations) differ from continuous media in that the content of the document is split into one or more discrete static display surfaces. To handle pages, CSS3 Paged Media describes how:

• page breaks are created and avoided;
• the page properties such as size, orientation, margins, border, and padding are specified;
• headers and footers are established within the page margins;
• content such as page counters are placed in the headers and footers; and
• orphans and widows can be controlled.

This module defines a page model that specifies how a document is formatted within a rectangular area, called the page box, that has finite width and height.

Although CSS3 does not specify how user agents transfer page boxes to sheets, it does include certain mechanisms for telling user agents about the intended page sheet size and orientation. In the general case, CSS3 assumes that one page box will be transferred to one surface of similar size.

All properties defined in this specification also accept the inherit keyword as their value, but for readability it has not been listed explicitly.

Page Terminology

The following terminology and accompanying diagrams help to describe the page model:

Page sheet

The page sheet is one surface of the physical medium. The illustration to the right shows a representation of the upper-left corner of a page sheet.

Printable and non-printable areas

The non-printable area is the area of a page sheet that a physical device such as a printer is not capable of marking reliably, usually due to the printer's paper handling mechanism. This value is printer dependent and is usually a small region along each edge of the page sheet. The printable area is the area of page sheet that a printer is capable of marking reliably. The size of the printable area is the size of the page sheet reduced by the size of the non-printable area. A user agent may not know the dimensions of this area for a particular printing device; but when its dimensions are known, user agents MAY adjust the formatting of the document so that content falls within the printable area. How this adjustment is accomplished is device dependent within the constraints expressed in the sections Rendering page boxes that do not fit a page sheet and Content outside the page box.

Page Orientation

The page orientation is defined by comparing the length of the edges of a page box. The page box is a rectangle with two perpendicular edges called the long edge and the short edge. The length of the long edge is always greater than or equal to the length of the short edge. When the page box is square, the two edges are of the same length and either can be used as the long edge with the other being the short edge. This specification defines page orientations of ''portrait'' and ''landscape''.

Portrait Orientation

A portrait page's height is greater than or equal to its width. Horizontal elements are parallel to the short edge and vertical elements to the long edge.

Landscape Orientation

A landscape page's width is greater than or equal to its height. Horizontal elements are parallel to the long edge and vertical elements to the short edge. Note that CSS3 makes no distinction between landscape and reverse-landscape orientations. However, future versions of CSS may do so. UAs should consider, when formatting for duplexed printing, the binding edge, page progression, and ease of reading when choosing between landscape and reverse-landscape renderings.

Duplex Printing

Duplex printing prints one page box per side of a page sheet and uses both sides of the page sheet. This module provides no ability to specify whether a document is duplex printed, but the concept of left and right pages is based on the assumption that the document is duplex printed, regardless of whether or not it actually is.

Binding Edge

The binding edge is the edge of the page box that is toward the binding if the material is bound. The binding edge often has a larger margin than the opposite edge to provide for the space used by the binding. The binding edge can be any of the four edges. However, page sheets are customarily bound so that the binding edge of page boxes with portrait orientation is vertical. This module provides no method to specify the binding edge. In duplex printing, the binding edge is on opposite sides of the page box for the left and right pages.

Facing Pages

Facing pages are two sequential pages such that when the document is duplex printed they are on separate sheets of paper. Typically, the earlier page will be the back side of one sheet and the later page will be the front side of another. They are usually laid out so that the binding edges of facing pages are vertical and adjacent when the pages are placed in their normal reading orientation.

Left Page

A page that would be on the left if it is part of a pair of facing pages as typically laid out. Page layouts for documents using a left-to-right page progression have the earlier of the facing pages on the left. Rules specific to the left page can be specified using the '':left'' page selector.

Right Page

A page that would be on the right if it is part of a pair of facing pages as typically laid out. Page layouts for documents using a right-to-left page progression have the earlier of the facing pages on the right. Rules specific to the right page can be specified using the '':right'' page selector.

The Page Model

In the paged media formatting model, the document is transferred into one or more page boxes. The page box is a specialized CSS box that maps to a rectangular print media surface, such as a page of paper. It is roughly analogous to the viewport.

As with other CSS boxes, a page box consists of margin, border, padding, and content areas. The content and margin areas of a page box have special functions:

In CSS 2.1, both the page box and page area are simple rectangles. Neither is a CSS box with margins, borders, and padding. This CSS box should be distinct from the page box and page area, which would be its margin area and content area, respectively. Naming ideas?

• The content area of a page box is called the page area. The content of the document is flowed into one or more page boxes. The page area acts as a container for all the boxes generated by the root element and its descendants that are laid out within a given page box. The edges of the page area on the first page establish the rectangle that is the initial containing block of the document.
• The margin area of a page box is divided into 16 page-margin boxes. Each page-margin box has its own margin, border, padding and content areas. Page-margin boxes are typically used to display running headers and footers.

The properties of a page box are determined by properties declared within the page context, which is the declaration block of the @page rule. Similarly the properties of a page-margin box are determined by properties declared within its margin context. Declarations in the page context can affect the page box and/or inherit to the page-margin boxes, but they do not apply to or inherit into the document's root element or other content.

The containing block of the page box is specified using the 'size' property in the page context. The width and horizontal margins of the page box are then calculated exactly as for a non-replaced block element in normal flow. [[!CSS21]] The height and vertical margins of the page box are calculated analogously (instead of using the block height formulas). In both cases if the values are over-constrained, instead of ignoring any margins, the containing block is resized to coincide with the margin edges of the page box.

Page Backgrounds and Painting Order

When drawing a page of content, the page layers are painted in the following painting order (bottommost first):

1. page background
2. document canvas
3. page borders
4. document contents
5. page-margin boxes

In the page model, the page background behaves similar to the root background: its background painting area is the bleed area, which covers the entire page box, including its margins (regardless of 'background-clip'). Page backgrounds are anchored within the page box's padding area by default (and honor 'background-origin' if the UA supports [[!CSS3BG]]). However if 'background-attachment' is ''fixed'' then the image is positioned relative to the page box including its margins (i.e. the background positioning area is the page's margin box).

The document canvas background is drawn as the page box's background: by default its background painting area covers the page box's border box, and for UAs that support [[!CSS3BG]], follows the 'background-clip' value specified on the root element. It remains, however, positioned with respect to the root element or page area as usual.

With respect to the page-margin boxes, the document canvas, page borders, and all of the document contents are treated as a single element with a z-index value of '0' that establishes a stacking context [[!CSS21]]: the page-margin boxes never interleave with parts of the document content or between the content and the canvas. They may only paint in front of the document content or behind the document canvas. The page background is always painted underneath everything else.

The 'z-index' property applies to page-margin boxes. Since the 'position' property does not apply to page-margin boxes, 'z-index' always affects page-margin boxes as if they were positioned elements regardless of the 'position' property's value. Each page-margin boxes always establishes a stacking context.

The default painting order, or CSS2.1 Appendix E "tree order", of page-margin boxes with respect to each other is as follows:

1. ''@top-left-corner''
2. ''@top-left''
3. ''@top-center''
4. ''@top-right''
5. ''@top-right-corner''
6. ''@right-top''
7. ''@right-middle''
8. ''@right-bottom''
9. ''@bottom-right-corner''
10. ''@bottom-right''
11. ''@bottom-center''
12. ''@bottom-left''
13. ''@bottom-left-corner''
14. ''@left-bottom''
15. ''@left-middle''
16. ''@left-top''

Start with ''@top-left-corner'', then go clockwise. This order is arbitrary but can be overridden with 'z-index'. It only has a visible effect when page-margin boxes overlap, which should not happen in most cases.

Content outside the page box

When formatting content in the page model, some content may end up outside the page box. For example, an element whose 'white-space' property has the value ''pre'' can generate a box that is wider than the page box. As another example, when boxes are positioned absolutely or relatively, they may end up in "inconvenient" locations. For example, images may be placed on the edge of the page box or 100,000 meters below the page box.

A specification for the exact formatting of such elements lies outside the scope of this document. However, it is recommended that authors and user agents observe the following general principles concerning content outside the page box:

• Content should be allowed slightly beyond the page box to allow pages to "bleed".

• User agents SHOULD avoid generating a large number of content-empty pages to honor the positioning of elements (e.g., printing 100 blank pages is probably neither the author's nor the user's intent). A Content-empty page is a page box whose page area contains no printable content other than backgrounds and/or borders. A page box whose page area contains generated content, or content whose visibility is ''hidden'', or invisible content such as a zero-width space is not a content-empty page. On the other hand, a page containing only a background and/or borders and/or page-margin box content is a content-empty page.

  Note, however, that generating a small number of empty page boxes is sometimes necessary to honor the forced-break values for 'page-break-before'/'break-before' and 'page-break-after'/'break-after'. [[!CSS21]] [[!CSS3-BREAK]]

• Authors SHOULD NOT position elements in inconvenient locations just to avoid rendering them. Instead:

  • To suppress box generation entirely, set the 'display' property to ''none''.
  • To make a box invisible, set the 'visibility' property.

• This specification does not define how boxes positioned outside the page box are handled. Possibilities include discarding them or creating page boxes for them at the end of the document.

Page Progression

CSS distinguishes between left pages and right pages on all documents, whether they are printed duplex or not. Each left page is followed by a right page and vice versa. Left and right pages can be styled differently with the :left and :right pseudo-classes.

Whether the first page of a document is a left page or a right page depends on the page progression of the document. The page progression is the direction in which the printed pages of a document would be sequenced when laid out side-to-side. For example, English and horizontally-set Japanese typically progress from left to right, whereas Arabic and vertically-set Japanese pages typically progress from right to left. In documents with a left-to-right page progression the first page of the document is a right page, and vice versa.

The page progression direction is determined as follows:

• If text is laid out in horizontal lines, the page progression is the same as the inline progression.
• If text is laid out in vertical lines, the page progression is the same as the block progression.

If the UA supports the 'direction' and 'writing-mode' properties from the CSS 3 Writing Modes Module [[CSS3-WRITING-MODES]], it must determine whether the first page is a left or right page from the values of those properties on the root element.

To explicitly force a document to begin printing on a left or right page, authors can specify a 'break-before' value that that propagates a page break to the root. [[!CSS3-BREAK]] The UA must suppress the first (empty) page(s) in this case (and the :first pseudo-class matches the first printed page).

html { break-before: always }

html { break-before: left }

Page Selectors and the Page Context

The @page Rule

Authors can specify various aspects of a page box, such as its dimensions, orientation, and margins, within an ''@page'' rule. ''@page'' rules are allowed wherever rule-sets are allowed. An ''@page'' rule consists of the keyword ''@page'', an OPTIONAL comma-separated list of page selectors and a block of declarations (said to be in the page context). An ''@page'' rule can also contain other at-rules, interleaved between declarations. The current level of this specification only allows margin at-rules inside ''@page''.

''@page'' rules without a selector list apply to every page. Other ''@page'' rules apply to pages that match at least one of their selectors. Properties declared within the page context apply to the page box.

If an error is encountered during the processing of a declaration block within a page or a margin context, the Rules for handling parsing errors apply; that is, valid declarations within the block are applied.

Page selectors

A page selector is made of either a page type selector or a page pseudo-class, followed by zero or more additional page pseudo-classes. No whitespace is allowed between components of a selector. The page selector grammar and examples can be found below.

A page selector is said to match a given page if and only if all of its components match the page.

A page type selector is a case-sensitive CSS identifier [[!CSS21]]. It matches pages of the named page type generated by the 'page' property. A page type name of ''auto'' (ASCII case-insensitive) does not make the rule invalid, but MUST never match.

A page pseudo-class is ASCII case-insensitive and has the same syntax as pseudo-classes in regular Selectors. [[!SELECT]] The various page pseudo-classes are defined below.

Spread pseudo-classes: '':left'', '':right''

When printing double-sided documents, left and right pages are often formatted differently. This can be expressed by using the '':left'' and '':right'' page pseudo-classes.

All pages are automatically classified by user agents as either left pages or right pages, based on page progression. The :left and :right pseudo-classes only match left or right pages, respectively.

@page :left {
margin-left: 3cm;
margin-right: 4cm;
}

@page :right {
margin-left: 4cm;
margin-right: 3cm;
}

If different declarations have been given for left and right pages, the user agent MUST honor these declarations even if the user agent does not transfer the page boxes to left and right sheets (i.e., a printer that only prints on one side of the medium must nevertheless produce correctly formatted output).

First-page pseudo-class: '':first''

The :first pseudo-class matches the first printed page of a document.

@page { margin: 2cm } /* All margins set to 2cm */

@page :first {
margin-top: 10cm /* Top margin on first page 10cm */
}

Blank-page pseudo-class: '':blank''

The :blank pseudo-class matches content-empty pages that appear as a result of forced page breaks.

h1 { break-before: left }

@page :blank {
  @top-center { content: "This page is intentionally left blank" }
}

A page matched by '':blank'' can also be matched by other page pseudo-classes.

h1 { break-before: left }

@page :blank {
  @top-center { content: none }
}

@page :right {
  @top-center { content: "Preliminary edition" }
  @bottom-center { content: counter(page) }
}

@page rule grammar

The syntax for the @page rule is a specialization of the generic at-rule defined by CSS 2.1. This grammar extends the at-rule syntax to allow @page rules nested inside @media rules. User agents MUST adhere to the following grammar:

See [[!CSS21]], Section 4.1.1 and Appendix G for the expansion of missing productions:

All new lexical tokens are specializations of the ATKEYWORD lexical token:

PAGE_SYM              ::= "@page"
TOPLEFTCORNER_SYM     ::= "@top-left-corner"
TOPLEFT_SYM           ::= "@top-left"
TOPCENTER_SYM         ::= "@top-center"
TOPRIGHT_SYM          ::= "@top-right"
TOPRIGHTCORNER_SYM    ::= "@top-right-corner"
BOTTOMLEFTCORNER_SYM  ::= "@bottom-left-corner"
BOTTOMLEFT_SYM        ::= "@bottom-left"
BOTTOMCENTER_SYM      ::= "@bottom-center"
BOTTOMRIGHT_SYM       ::= "@bottom-right"
BOTTOMRIGHTCORNER_SYM ::= "@bottom-right-corner"
LEFTTOP_SYM           ::= "@left-top"
LEFTMIDDLE_SYM        ::= "@left-middle"
LEFTBOTTOM_SYM        ::= "@left-bottom"
RIGHTTOP_SYM          ::= "@right-top"
RIGHTMIDDLE_SYM       ::= "@right-middle"
RIGHTBOTTOM_SYM       ::= "@right-bottom"

media :
     MEDIA_SYM S* medium [ COMMA S* medium ]* LBRACE S* [ page_rule | ruleset ]* '}' S*
     ;

page_rule :
     PAGE_SYM S* page_selector_list '{' S* page_body '}' S*
     ;

page_selector_list :
     [ page_selector S* [ ',' page_selector S* ]* ]?
     ;

page_selector :
     pseudo_page+ | IDENT pseudo_page*
     ;

pseudo_page :
     ':' [ "left" | "right" | "first" | "blank" ]
     ;

page_body :  /* Can be empty */
     declaration? [ ';' S* page_body ]? |
     page_margin_box page_body
     ;

page_margin_box :
     margin_sym S* '{' S* declaration? [ ';' S* declaration? ]* '}' S*
     ;

margin_sym :
     TOPLEFTCORNER_SYM |
     TOPLEFT_SYM |
     TOPCENTER_SYM |
     TOPRIGHT_SYM |
     TOPRIGHTCORNER_SYM |
     BOTTOMLEFTCORNER_SYM |
     BOTTOMLEFT_SYM |
     BOTTOMCENTER_SYM |
     BOTTOMRIGHT_SYM |
     BOTTOMRIGHTCORNER_SYM |
     LEFTTOP_SYM |
     LEFTMIDDLE_SYM |
     LEFTBOTTOM_SYM |
     RIGHTTOP_SYM |
     RIGHTMIDDLE_SYM |
     RIGHTBOTTOM_SYM
     ;

@page { ... }
@page :left { ... }
@page :right { ... }
@page LandscapeTable { ... }
@page CompanyLetterHead:first { ... } /*  identifier and pseudo page. */
@page:first { ... }
@page toc, index { ... }
@page :blank:first { ... }

@page {
 @top-left { ... /* document name */ }
 @bottom-center { ... /* page number */}
}
@page :left { @left-middle { ... /* page number in left margin */ }}
@page :right{ @right-middle { ... /* page number in right margins of right pages */}}

@page :left { @bottom-left-corner { ... /* left page numbers */ }}
@page :right { @bottom-right-corner { ... /* right page numbers */ }}
@page :first { @bottom-left-corner { ... /* empty footer on 1st page */ }
             @bottom-right-corner { ... /* empty footer */ } }

Cascading in the page context

Declarations in page and margin contexts cascade just like declarations in style rule for elements.

The specificity of page a selector is computed in a manner analogous to the computations defined in the Selectors module:

• Count the number of page type names (= f)

  Given the syntax of page seletors, f can only ever be 0 or 1.

• Count the number of ':first' or ':blank' pseudo-classes (= g)
• Count the number of ':left' or ':right' pseudo-classes (= h)

Note: Repeated occurrences of the same pseudo-classes are allowed and do increase specificity.

Due to storage limitations, implementations may have limitations on the size of f, g, or h. If so, values higher than the limit must be clamped to that limit, and not overflow.

Specificities are compared by comparing the three components in order (f, g, h): the specificity with a larger f value is more specific; if the two f values are tied, then the two g values are compared, etc. If all the values are tied, the two specificities are equal.

@page { } /* specificity = (0,0,0) */
@page :left { } /* specificity = (0,0,1) */
@page :first { } /* specificity = (0,1,0) */
@page :blank:left { } /* specificity = (0,1,1) */
@page artsy { } /* specificity = (1,0,0) */
@page artsy:left { } /* specificity = (1,0,1) */
@page artsy:first { } /* specificity = (1,1,0) */

@page :left {
  margin-left: 4cm;
}

@page {
  margin-left: 3cm;
}

@page :first {
  color: green;

  @top-left {
    content: "foo";
    color: blue;
  }
  @top-right {
    content: "bar";
  }
}

@page { color: red;
  @top-center {
    content: "Page " counter(page);
  }
}

@page { margin: 25mm;}
@page { font-size: 14pt;}

Page-Margin Boxes

Page-margin boxes are boxes within the page margin that, like pseudo-elements, can contain generated content.

Page-margin boxes can be used to create page headers and footers, which are portions of the page set aside for supplementary information such as the page number or document title.

Page-margin boxes are positioned with respect to the page area and are independent of page orientation, for example the top page-margin boxes are above the page area in both portrait and landscape orientation. The various page-margin boxes are defined and illustrated in the diagram below:

Table 1 Page-Margin Box Definitions

Box	Description	Placement
top-left-corner	a fixed-size box defined by the intersection of the top and left margins of the page box
top-left	a variable-width box filling the top page margin between the top-left-corner and top-center page-margin boxes
top-center	a variable-width box centered horizontally between the page's left and right border edges and filling the page top margin between the top-left and top-right page-margin boxes
top-right	a variable-width box filling the top page margin between the top-center and top-right-corner page-margin boxes
top-right-corner	a fixed-size box defined by the intersection of the top and right margins of the page box
left-top	a variable-height box filling the left page margin between the top-left-corner and left-middle page-margin boxes
left-middle	a variable-height box centered vertically between the page's top and bottom border edges and filling the left page margin between the left-top and left-bottom page-margin boxes
left-bottom	a variable-height box filling the left page margin between the left-middle and bottom-left-corner page-margin boxes
right-top	a variable-height box filling the right page margin between the top-right-corner and right-middle page-margin boxes
right-middle	a variable-height box centered vertically between the page's top and bottom border edges and filling the right page margin between the right-top and right-bottom page-margin boxes
right-bottom	a variable-height box filling the right page margin between the right-middle and bottom-right-corner page-margin boxes
bottom-left-corner	a fixed-size box defined by the intersection of the bottom and left margins of the page box
bottom-left	a variable-width box filling the bottom page margin between the bottom-left-corner and bottom-center page-margin boxes
bottom-center	a variable-width box centered horizontally between the page's left and right border edges and filling the bottom page margin between the bottom-left and bottom-right page-margin boxes
bottom-right	a variable-width box filling the bottom page margin between the bottom-center and bottom-right-corner page-margin boxes
bottom-right-corner	a fixed-size box defined by the intersection of the bottom and right margins of the page box

At-rules for page-margin boxes

Page-margin boxes are created by margin at-rules inside the page context. Authors should put these rules after any declarations in the page context as legacy clients may not handle declarations after margin at-rules correctly.

A margin at-rule consists of an ATKEYWORD that identifies the page-margin box (e.g. ''@top-left'') and a block of declarations (said to be in the margin context).

@page {
  size: 8.5in 11in;
  margin: 10%;

  @top-left {
    content: "Hamlet";
  }
  @top-right {
    content: "Page " counter(page);
  }
}

Populating page-margin boxes

As with the '':before'' and '':after'' pseudo-elements, a specified 'content: normal' on a page-margin box computes to ''none''. A page-margin box is generated if and only if the computed value of its 'content' property is not ''none''. Otherwise, no box is generated, as for elements with ''display: none''.

The 'display' property does not apply to page-margin boxes.

  @page {
    @top-left-corner { content: " "; border: solid green; }
    @top-right-corner { content: url(foo.png); border: solid green; }
    @bottom-right-corner { content: counter(page); border: solid green; }
    @bottom-left-corner { content: normal; border: solid green; }
  }

Computing Page-margin Box Dimensions

The width and height of each page-margin box is determined by the rules below. These rules define the equivalent of CSS2.1 Sections 10.3 and 10.6 for page-margin boxes.

The rules for applying 'min-height', 'max-height', 'min-width', and 'max-width' [[!CSS21]] do apply to page-margin boxes and may imply a recalculation of the width, height, and/or margins if the dimensions resulting from the specified 'width' or 'height' violate their constraints. If the UA does not support the 'min-height' or 'min-width' properties then it must behave as if 'min-height' and 'min-width' were always zero.

Page-Margin Box Layout Terminology

In addition to the box model definitions in CSS2.1 [[!CSS21]], and the sizing terms in CSS Intrinsic Sizing [[!CSS3-SIZING]], the following terms are defined for use in the subsequent page-margin box calculations:

available width

The sum of the page’s left border width, left padding, page area width, right padding, and right border width. In other words, it is the distance between the page box’s left right border edges. This quantity is used when calculating dimensions of the top and bottom page-margin boxes.

available height

The sum of the page’s top border width, top padding, page area height, bottom padding, and bottom border width. In other words, it is the distance between the page box’s top bottom border edges. This quantity is used when calculating dimensions of the left and right page-margin boxes.

outer width

The width of the outer edge, as defined in [[!CSS21]].

min-content width

Whichever of min-content measure or min-content extent is the physical width.

max-content width

Whichever of max-content measure or max-content extent is the physical width.

outer min width

Like the outer width, except the min-content width is used when the computed 'width' is ''auto''.

outer max width

Like the outer width, except the max-content width is used when the computed 'width' is ''auto''.

The containing block for a page-margin box depends on its location:

For a corner page-margin box, it is the rectangle defined by the intersection of the two page margins meeting at that corner.

For all other page-margin boxes, the containing block is the rectangle formed by the encapsulating page margin minus the containing blocks of the adjacent corners' page-margin boxes. This means that the size of this containing block is given in one dimension by the used page margin and in the other dimension by the available width (for top and bottom page-margin boxes) or available height (for left and right page-margin boxes).

Page-Margin Box Variable Dimension Computation Rules

The following rules apply to ''@top-left'', ''@top-center'' and ''@top-right'' page-margin boxes, which are referred to as A, B, and C, respectively, in this section.

Margins

If the 'margin-left' or 'margin-right' property of any of the three boxes computes to ''auto'', the used value is zero.

Resolving ''auto'' widths

The following algorithm determines the used width of each box. For this purpose, boxes that are not generated are assumed to have a 'width' and an outer width of zero.

Note: The high-level goals are (in order of priority) to center the middle box (B) if it is generated, to minimize overflow and overlap, and to distribute space proportionally to the amount of content.

If the middle box (B) is not generated, distribute the available width to A and C as follows:

• If only one box has 'width: auto', its used width is resolved so that the sum of the outer widths equals available width.
• If A and C both have 'width: auto', distribute the space to each box as follows:
  1. If the sum of the outer max-content widths is less than the available width, call that difference the flex space. Calculate each box's flex factor as proportional to its outer max-content width, and set its used outer width to:

     max-content width + flex space × flex factor ÷ ∑flex factors

  2. Otherwise if the sum of the outer min-content widths is less than the available width, call that difference the flex space calculate each box's flex factor as proportional to its max-content width minus min-content width, and set its used outer width to:

     min-content width + flex space × flex factor ÷ ∑flex factors

  3. Otherwise, calculate its outer size as in the previous case, but set each box's flex factor as proportional to its outer min-content width.
  In each case, both flex factors are assumed to be ''1'' if their sum is equal to zero.

If the middle box (B) is generated, determine the ''auto'' widths of A, B, and C as follows:

1. First, resolve any ''auto'' width of the middle box (B): Assume there are two boxes, B and AC, where each of AC's dimensions is double the maximum of A and C. (This preserves B's centering.) Distribute the space to these two boxes (B and the imaginary AC) as described for A and C above.
2. Then, resolve any ''auto'' widths of the side boxes (A and C) by setting that box's outer width to

   (available width − used outer widths of B) ÷ 2

Handling 'min-width' and 'max-width'

The 'min-width' and 'max-width' properties [[CSS21]] apply to page-margin boxes in the variable dimension like on normal elements, except that the three boxes on the same side are considered together.

More precisely:

1. The tentative used widths are calculated (without 'min-width' and 'max-width') following the rules under Resolving ''auto'' widths above.
2. If the tentative used width of any of the three boxes is greater than 'max-width', the rules above are applied again, but this time using the computed value of 'max-width' as the computed value for 'width'.
3. If the resulting width of any of the three boxes is smaller than 'min-width', the rules above are applied again, but this time using the value of 'min-width' as the computed value for 'width'.

Positioning

Once the dimensions of the boxes are determined, they are positioned as follows:

• The left outer edge of A is flush with the left edge of the containing block
• The outer area of B is centered in the containing block.
• The right outer edge of C is flush with the right edge of the containing block.

Boxes on other sides

The used values for ''@bottom-left'', ''@bottom-center'' and ''@bottom-right'' page-margin boxes are established by the same rules as for ''@top-left'', ''@top-center'', and ''@top-right'', respectively.

The used values for ''@left-top'', ''@left-middle'' and ''@left-bottom'' boxes are established by the same rules, with "width" replaced by "height", "left" by "top", "right" by "bottom" and "center" by "middle".

The used values for ''@right-top'', ''@right-middle'' and ''@right-bottom'' page-margin boxes are established by the same rules as for ''@left-top'', ''@left-middle'' and ''@left-bottom'', respectively.

Page-Margin Box Fixed Dimension Computation Rules

The rules below are used to calculate the used values of each ''@top-left-corner'', ''@top-left'', ''@top-center'', ''@top-right'', and ''@top-right-corner'' page-margin box's 'height', 'margin-top', and 'margin-bottom' properties:

1. The following constraint must hold among the used values of the margin box's properties:

   'margin-top' + 'border-top-width' + 'padding-top' + 'height' + 'padding-bottom' + 'border-bottom-width' + 'margin-bottom' = top page margin

2. If 'border-top-width' + 'padding-top' + 'height' (if it is not ''auto'') + 'padding-bottom' + 'border-bottom-width', plus 'margin-top' and/or 'margin-bottom' if not ''auto'', is larger than the height of the top page margin, then any ''auto'' values for 'margin-top' or 'margin-bottom' are, for the following rules, treated as zero.
3. If at this point all of 'height', 'margin-top', and 'margin-bottom' have a computed value other than ''auto'', the values are said to be "over-constrained". In this case, the specified value of 'margin-top' is treated as ''auto''.
4. If there is now exactly one value specified as ''auto'', its used value follows from the equality.
5. If 'height' is set to ''auto'', any other ''auto'' values become '0' and 'height' follows from the resulting equality
6. If both 'margin-top' and 'margin-bottom' are ''auto'', their used values are equal. This vertically centers the page-margin box content within the top page margin.

The same rules apply to the bottom page-margin boxes (bottom-left-corner, bottom-left, bottom-center, bottom-right, and bottom-right-corner), except that in the overconstrained case, the 'margin-bottom' is ignored rather than the 'margin-top'.

Analogous rules govern the properties for the left and right page-margin boxes with respect to 'width' (top-left-corner, left-top, left-middle, left-bottom, and bottom-left-corner; top-right-corner, right-top, right-middle, right-bottom, bottom-right-corner), with "top" replaced by "left", "bottom" replaced by "right", and "height" replaced by "width". In the overconstrained case for left (right) page-margin boxes, the specified value of 'margin-left' ('margin-right') is ignored.

Page-margin box examples

The following is a collection of examples of page-margin box usage.

@page {
@top-left { content: "Header in Left Cell (top-left)" }
}

@page {
@top-center { content: "Header in Center Cell (top-center)" }
}

@page {
@top-right { content: "Header in Right Cell (top-right)" }
}

@page {
@top-left { content: "Left Cell (top-left)" }
@top-center { content: "Header in Center Cell (top-center)" }
}

@page {
@top-center { content: "Header in Center Cell (top-center)" }
@top-right { content: "Right Cell (top-right)" }
}

@page {
@top-left { content: "Header in top-left with approx. "
                   "twice as many words as right cell." }
@top-right { content: "Right cell (top-right)" }
}

Page Properties

@page {
  margin: 10%;

  @top-center {
    font-family: sans-serif;
    font-weight: bold;
    font-size: 2em;
    content: counter(page);
  }
}

@page {
  counter-increment: page 2;
}

body {counter-reset: chapter;}
div.chapter {counter-increment: chapter;}
@page {
margin: 10%;
@top-center { content: "Chapter" counter(chapter) }
}

  @page {
    size: 4in 6in;
  }

  @media (max-width: 6in) {
    @page {
      size: letter;
    }
  }
  

@page {
size: A4 landscape;
}

@page {
size: auto;/* auto is the initial value */
margin: 10%;
}

@page {
size: 8.5in 11in;/* width height */
}

 /* style sheet for "A4" printing */
 @media print and (width: 21cm) and (height: 29.7cm) {
    @page {
       margin: 3cm;
    }
 }

 /* style sheet for "letter" printing */
 @media print and (width: 8.5in) and (height: 11in) {
    @page {
        margin: 1in;
    }
 }

	Name: marks
	Value: none | [ crop || cross ]
	Initial: none
	Applies To: page context
	Inherited: no
	Computed Value: as specified
	Media: paged
	

@page { marks: crop cross }

	Name: bleed
	Value: auto | <>
	Initial: auto
	Applies To: page context
	Inherited: no
	Percentages: N/A
	Computed Value: as specified
	Media: paged
	

@page narrow { size: 9cm 18cm }
@page rotated { size: landscape }
div { page: narrow }
table { page: rotated }

<div>
<table>...</table>
<table>...</table>
<p>This text is rendered on a 'narrow' page</p>
</div>

<!DOCTYPE html>
<html lang="ja">
  <style>
    html   { writing-mode: vertical-rl;
             line-height: 1.6; }
    .main  { page: main;
             columns: 2; column-gap: 1rem; }
    .index { page: index;
             columns: 3; column-gap: 1rem; }
    @page       { margin: auto;    /* center kihon hanmen on page */
                  width:  40rem; } /* 1.6 × 25 lines        */
    @page main  { height: 61rem; } /* 2 × 30 chars + 1 × gap */
    @page index { height: 62rem; } /* 3 × 20 chars + 2 × gap */
  </style>
  <section class="main"> ... </section>
  <section class="index"> ... </section>
</html>