csswg-drafts/css2/visudet.src at 017ad6174594e9809b31ae2179d5242c7250dd0b · w3c/csswg-drafts · GitHub

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html lang="en">
<!-- $Id: visudet.src,v 2.25 1998-04-04 00:30:19 ijacobs Exp $ -->
<HEAD>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<TITLE>Visual rendering model details</TITLE>
</HEAD>
<BODY>
<H1 align="center">Visual rendering model details</H1>

<H2><a name="containing-block-details">Details of containing block
calculation</a></H2>

<P>The containing block for a box other than the root box is determined
as follows:</p>

<ul>
<li>If the value of the <span
class="propinst-position">'position'</span> property for the parent
element is anything but 'static' then the containing block is established
by the parent box.
<li>Otherwise, if the value of the <span
class="propinst-display">'display'</span> property for the parent
element is anything but 'inline' then the containing block is
established by the parent box.
<!-- What about display: none above? -IJ -->
<li>Otherwise, the containing block is the parent's containing block.
</ul>

<P>When a containing block is established by a <a
href="visuren.html#block-box">block</a> box, it has the same
width, height, and position as the box's <a
href="box.html#padding-edge">padding edge</a>. The edges
are called the top, bottom, left, and right of the containing block.

<P>When a containing block is established by an <a
href="visuren.html#inline-level">inline-level</a> element, it has the
same width, height, and position as the <a
href="box.html#padding-edge">padding edge</a> of the first (in the
source tree) inline box generated by the inline-level element.

<!-- The previous paragraph does not comply with the WG's decision
in. Note, especially, in WG solution, that percentages applied to
negative widths cause problems. -IJ -->

<P><a href="visuren.html#relative-positioning">Relatively
positioned</a> <a href="visuren.html#inline-box">inline</a> boxes must be
considered specially since (1) the <a
href="conform.html#rendered-content">rendered content</a> of an inline-level
element may not be rectangular in shape but (2) a relatively
positioned box establishes a new containing block and containing
blocks must be rectangular. The reference edges for descendants
are the following:</p>

<ul>
<li>For left-to-right scripts, the left and top edges are those
of the first inline box. The bottom and right edges are those
of the last inline box.
<li>For right-to-left scripts, the right and top edges are those
of the first inline box. The bottom and left edges are those
of the last inline box.
</ul>

<H2>Calculation of box and line box widths</h2>
<P>Box and line box widths and heights may be established in one of
several ways:</p>

<dl>
<dt><em>Width and height properties set explicitly</em>

<dd>If <span class="propinst-width">'width'</span> or <span
class="propinst-height">'height'</span> is set explicitly (i.e., not
'auto'), the box will have the specified width or height, except in
tables, where a 'width' or 'height' is interpreted as a
<em>minimum</em> size, or even ignored, depending on the <a
href="tables.html#layout-algos">table layout algorithm</a>.
The <span class="propinst-width">'width'</span>
and <span class="propinst-height">'height'</span> properties
do not apply to non-replaced inline elements.
<dt><em>Block box widths are calculated top-down</em>
<dd>The
<a href="#box-width">width of a block box</a>
is given by the width of its
containing block minus the box's margins, borders, and padding.
<dt><em>Inline box widths are calculated bottom-up</em>
<dd>The <a
href="#box-width">width of an inline box</a>
is given by its
<a href="conform.html#rendered-content">rendered content</a>.
<dt><em>Block box heights are calculated bottom-up</em>
<dd>Block level boxes grow to the size of the boxes
they contain (unless the <span class="propinst-height">'height'</span>
property is set).
Please consult the section on <a
href="#box-height">box height calculations</a> for
details.
<dt><em>Inline box heights are given by the <span
class="propinst-line-height">'line-height'</span> property</em>
<dd>Please consult the section on <a
href="#box-height">box height calculations</a> for
details.
<dt><em>Line heights depend on inline box heights and alignment</em>
<dd>Please consult the section on <a
href="#line-height">line height calculations</a> for
details.
<dt><em><a name="intrinsic-dim">Intrinsic dimension of replaced
content</a></em>
<dd>The <a href="conform.html">rendered content</a> of a <a
href="conform.html#replaced-element">replaced element</a> may have
"intrinsic dimensions" that user agents use as the <a
href="cascade.html#computed-value">computed</a> content
width and height (e.g., the unscaled width and height of an included
image). If the intrinsic dimensions are overridden, the
replaced content is scaled by the user agent.
When scaling an element with intrinsic dimensions,
the aspect ratio is preserved
if values for the <span class="propinst-width">'width'</span> and
<span class="propinst-height">'height'</span> properties are set to
'auto'.
<dt><em>Hybrid calculations</em>
<dd>The dimension of a table cell is determined by both the cell's
contents and the surrounding available space.
</dl>

<P>The sections below describe these calculations in detail.

<H2><a name="box-width">Box width calculations</a></H2>

<P>As discussed in the section on <a
href="box.html#box-dimensions">box dimensions</a>, the <a
href="box.html#content-width">content width</a> of a box is assigned
explicitly (via the <span class="propinst-width">'width'</span>
property), given "intrinsically" (for replaced elements), or
calculated "top-down" (based on the width of the box's containing
block).

<P>The following sections explain the exact computation of a box's
width-related properties.

<h3><a name="the-width-property">Content width</a>: the <span
class="propinst-width">'width'</span> property</H3>

<!-- #include src=properties/width.srb -->

<P> This property specifies the <a
href="box.html#content-width">content width</a> of a block box. Values
have the following meanings:</P>

<dl>
<dt><span class="index-inst" title="&lt;length&gt;"><span class="value-inst-length"><strong>&lt;length&gt;</strong></span></span>
<dd>Specifies a fixed width.
<dt><span class="index-inst" title="&lt;percentage&gt;"><span class="value-inst-percentage"><strong>&lt;percentage&gt;</strong></span></span>
<dd>Specifies a percentage width. The percentage is calculated
with respect to width
of the generated box's
<a href="visuren.html#containing-block">containing block</a>.
<dt><strong>auto</strong>
<dd>See the section on <a href="#normal-floated">widths
of boxes in the normal flow
and floated boxes</a>.
</dl>

<P>Negative values for <span class="propinst-width">'width'</span> are
illegal. User agents must <a href="syndata.html#ignore">ignore</a> this
property for elements with <span
class="propinst-display">'display'</span> set to 'row' or 'row-span'.

<div class="example"><P>
For example, the following rule fixes the width of paragraphs
to 100 pixels:</p>

<PRE>
P { width: 100px }
</PRE>
</div>

<h3><a name="normal-floated">Width of boxes in the normal flow and
floated boxes</a></h3>

<P>The <span class="propinst-width">'width'</span> property does not
apply to non-replaced <a
href="visuren.html#inline-level">inline-level</a> elements. These
elements generate boxes that flow into <a
href="visuren.html#line-box">line boxes</a>. The width of line boxes
is given by the their <a
href="visuren.html#containing-block">containing block</a>, but may be
shorted by the presence of <a href="visuren.html#floats">floats</a>.

<P>For other types of boxes in the <a
href="visuren.html#normal-flow">normal flow</a> and for <a
href="visuren.html#floats">floats</a>, a box's margin, padding,
border, and content width must equal the width of its <a
href="visuren.html#containing-block">containing block</a>. These widths are
specified with the following seven properties: <span
class="propinst-margin-left">'margin-left'</span>, <span
class="propinst-border-left">'border-left'</span>, <span
class="propinst-padding-left">'padding-left'</span>, <span
class="propinst-width">'width'</span>, <span
class="propinst-padding-right">'padding-right'</span>, <span
class="propinst-border-right">'border-right'</span>, and <span
class="propinst-margin-right">'margin-right'</span>.
However, if the <span class="propinst-border-style">'border-style'</span>
for an element has the value 'none', the corresponding border widths
will be '0'.


<P>Three of these properties (<span
class="propinst-margin-left">'margin-left'</span>, <span
class="propinst-width">'width'</span>, and <span
class="propinst-margin-right">'margin-right'</span>) may take the
value 'auto'. Their <a href="cascade.html#computed-value">computed
values</a> are determined as follows.

<h4>Determining the content width</h4>

<P>When the <span class="propinst-width">'width'</span>
property has the value 'auto', user agents should
assign it a <a href="cascade.html#computed-value">computed value</a>
based on <a href="#comp-margin-widths">computed margin widths</a>.
In addition:</p>

<ol>
<li>For <a href="visuren.html#inline-level">inline-level</a> elements
and those that generate <a href="visuren.html#floats">floating</a>
boxes, the computed value of <span
class="propinst-width">'width'</span> is '0'.

<li>For <a href="conform.html#replaced-element">replaced elements</a>,
user agents should use the <a
href="#intrinsic-dim">intrinsic width</a> of the element's
<a href="conform.html#rendered-content">rendered content</a> as the
box's <a href="cascade.html#computed-value">computed</a> width.  (Note
that for other values of <span class="propinst-width">'width'</span>,
the replaced contents may be scaled).

<li><a href="#min-max-widths">Minimum and maximum</a> computed values
for the <span class="propinst-width">'width'</span> property may be
assigned with the <span class="propinst-min-width">'min-width'</span>
and <span class="propinst-max-width">'max-width'</span> properties.
</ol>

<h4><a name="comp-margin-widths">Computing margin widths</a></h4>

<P>User agents should assign <a
href="cascade.html#computed-value">computed values</a> for <span
class="propinst-margin-left">'margin-left'</span>, <span
class="propinst-width">'width'</span>, and <span
class="propinst-margin-right">'margin-right'</span> as follows:</p>

<ol>

<li>If exactly one of <span
class="propinst-margin-left">'margin-left'</span>, <span
class="propinst-width">'width&#
FE6D
039;</span> or <span
class="propinst-margin-right">'margin-right'</span> has the value
'auto', the computed value of that property is assigned
so that the seven width properties add up to the
width of the <a href="visuren.html#containing-block">containing block</a>.

<li>If none of the properties has the value 'auto', <span
class="propinst-margin-right">'margin-right'</span> is assigned a
computed value so that the seven width properties add up to the
width of the <a href="visuren.html#containing-block">containing block</a>.

<li>If more than one of the three has the value 'auto', and one of
them is <span class="propinst-width">'width'</span>, than the other
two (<span class="propinst-margin-left">'margin-left'</span> and/or
<span class="propinst-margin-right">'margin-right'</span>) will be
assigned the computed value '0' and <span
class="propinst-width">'width'</span> will be assigned a computed
value so that the seven width properties add up to the width of the <a
href="visuren.html#containing-block">containing block</a>.

<li>Otherwise, if both <span
class="propinst-margin-left">'margin-left'</span> and <span
class="propinst-margin-right">'margin-right'</span> have the value
'auto', they will each be assigned a computed value that is half of
the available horizontal space (this centers the box within its
containing block).
</ol>

<H3><a name="abs-width">Width of absolutely positioned boxes</a></H3>

<P>The width of an <a
href="visuren.html#absolute-positioning">absolutely positioned</a> box
may be computed in three ways:

<ol>
<li>The element generating the box has a
 value other than 'auto' for the <span
class="propinst-width">'width'</span> property.
<li>The element is a replaced element with an intrinsic width
and <span class="propinst-width">'width'</span> has the value 'auto'.
<li>The width is given by the difference between the
<span class="propinst-left">'left'</span> and
<span class="propinst-right">'right'</span> offsets
(minus padding, borders, and margins). When
padding, border, and margin properties have the specified
value 'auto', the computed value is '0'.
</ol>

<P>In cases one and two, for left-to-right formatting, values of <span
class="propinst-left">'left'</span> and <span
class="propinst-right">'right'</span> are computed as follows:</p>

<ul>
<li>If both <span class="propinst-left">'left'</span> and
<span class="propinst-right">'right'</span> have the value
'auto', <span class="propinst-left">'left'</span> is the
offset between the left containing block edge and
the box's left side as though it were in normal flow. Then <span
class="propinst-right">'right'</span> is computed based
on the known left offset and width.
<li>If one property has a value other than 'auto',
this known offset and the width are used to calculate
the offset of the opposite side.
<li>If both properties have a value other than 'auto', <span
class="propinst-right">'right'</span> is treated as though it
were 'auto'.
</ul>

<P>In case three, for left-to-right formatting, values of <span
class="propinst-left">'left'</span>, <span
class="propinst-right">'right'</span>, and <span
class="propinst-width">'width'</span> are computed as follows:</P>

<ul>
<li>If <span class="propinst-right">'right'</span> has
the value 'auto', it is assigned the value '0'.
The width is the (absolute) distance
between the right containing
block edge and the box's left edge. A
value of 'auto' for <span class="propinst-left">'left'</span>
means the left offset is calculated as though the box
were in normal flow.
<li>For other values of <span class="propinst-right">'right'</span>,
the width is the (absolute) distance
between the right offset
and the position of the box's left edge, calculated as before.
</ul>

<P>For all of the above cases, analogous rules hold for right-to-left
formatting, switching right and left.

<H3><a name="min-max-widths">Minimum and maximum widths</a>: <span
class="propinst-min-width">'min-width'</span> and <span
class="propinst-max-width">'max-width'</span></h3>

<!-- #include src=properties/min-width.srb -->
<!-- #include src=properties/max-width.srb -->

<P>These two properties allow authors to constrain box widths to a
certain range. Values have the following meanings:</p>

<dl>
<dt><span class="index-inst" title="&lt;length&gt;"><span class="value-inst-length"><strong>&lt;length&gt;</strong></span></span>
<dd>Specifies a fixed minimum or maximum computed width.
<dt><span class="index-inst" title="&lt;percentage&gt;"><span class="value-inst-percentage"><strong>&lt;percentage&gt;</strong></span></span>
<dd>Specifies a percentage for determining the computed value.
The percentage is calculated
with respect
to the width of the
generated box's
<a href="visuren.html#containing-block">containing block</a>.
</dl>

<P>The following algorithm describes how the two properties influence
the <a href="cascade.html#computed-value">computed value</a>
of the <span class="propinst-width">'width'</span> property:</p>

<OL>

<LI>the <a href="#normal-floated">normal width</a> is computed
(without <span class="propinst-min-width">'min-width'</span> and <span
class="propinst-max-width">'max-width'</span>).

<LI>if the computed value of <span
class="propinst-min-width">'min-width'</span> is greater than the
value of <span class="propinst-max-width">'max-width'</span>, <span
class="propinst-max-width">'max-width'</span> is set to the
value of <span class="propinst-min-width">'min-width'</span>.

<LI>if the computed width is greater than <span
class="propinst-max-width">'max-width'</span>, the computed value of
<span class="propinst-width">'width'</span> is set to the value of
<span class="propinst-max-width">'max-width'</span>.

<LI>if the computed width is smaller than <span
class="propinst-min-width">'min-width'</span>, the computed value of
<span class="propinst-width">'width'</span> is set to the value of
<span class="propinst-min-width">'min-width'</span>.

</OL>

<P>The user agent may define a non-negative minimum value for the
<span class="propinst-width">'width'</span> property, which may vary
from element to element and even depend on other properties. If <span
class="propinst-width">'width'</span> goes below this limit, either
because it was set explicitly, or because it was 'auto' and the rules
below would make it too small, the user agent may use the minimum
value as the computed value.

<H2><a name="box-height">Box height calculations</a></H2>

<P>As discussed in the section on <a
href="box.html#box-dimensions">box dimensions</a>, the <a
href="box.html#content-height">content height</a> of a box is assigned
explicitly (via the <span class="propinst-height">'height'</span>
property), given "intrinsically" (for replaced elements), or
calculated as the minimal height necessary to include the vertical
content of the element and that of all its flowed children. This is
the height necessary <em>before</em> any relative offset of children.

<P>The following sections explain the exact computation of a
box's height-related properties.

<h3><a name="the-height-property">Content height</a>: the <span
class="propinst-height">'height'</span> property</H3>

<!-- #include src=properties/height.srb -->

<P> This property specifies a box's <a
href="box.html#content-height">content height</a>. Values have the
following meanings:</P>

<dl>
<dt><span class="index-inst" title="&lt;length&gt;"><span class="value-inst-length"><strong>&lt;length&gt;</strong></span></span>
<dd>Specifies a fixed height.
<dt><span class="index-inst" title="&lt;percentage&gt;"><span class="value-inst-percentage"><strong>&lt;percentage&gt;</strong></span></span>
<dd>Specifies a percentage height. The percentage is calculated
with respect to the <em>width</em> of the
generated box's <a href="visuren.html#containing-block">containing block</a>.
<dt><strong>auto</strong>
<dd>See the section on <a href="#normal-floated">heights
of boxes in the normal flow
and floated boxes</a>.
</dl>

<P>Negative values for <span class="propinst-height">'height'</span>
are illegal. User agents must <a href="syndata.html#ignore">ignore</a>
this property for elements with <span
class="propinst-display">'display'</span> set to 'col' or 'column-span'.

<div class="example"><P>
For example, the following rule fixes the height of paragraphs
to 100 pixels:</p>

<PRE>
P { height: 100px }
</PRE>

<P>Paragraphs that require more than 100 pixels of height will
<a href="visufx.html#overflow">overflow</a> according
to the <span class="propinst-overflow">'overflow'</span> property.
</div>

<P>The height may be enforced by the user interface (e.g., a
scrollbar).

<H3><a name="normal-height">Determining the content height</a></H3>

<P>When the <span class="propinst-height">'height'</span> property has
the value 'auto', user agents should assign it a <a
href="cascade.html#computed-value">computed value</a> based on the
space required by the element's <a
href="conform.html#rendered-content">rendered content</a>.  In
addition:</p>

<ol>

<li>For <a href="conform.html#replaced-element">replaced elements</a>,
user agents should use the <a
href="#intrinsic-dim">intrinsic height</a> of the element's
<a href="conform.html#rendered-content">rendered content</a> as the
box's <a href="cascade.html#computed-value">computed</a> height.  (Note
that for other values of <span class="propinst-height">'height'</span>,
the replaced contents may be scaled).

<li><a href="#min-max-widths">Minimum and maximum</a> computed values
for the <span class="propinst-height">'height'</span> property may be
assigned with the <span class="propinst-min-height">'min-height'</span>
and <span class="propinst-max-height">'max-height'</span> properties.
</ol>

<H3>Height of absolutely positioned boxes</H3>

<P>The height of an <a
href="visuren.html#absolute-positioning">absolutely positioned</a> box
may be computed in three ways:

<ol>
<li>The element generating the box has a
 value other than 'auto' for the <span
class="propinst-height">'height'</span> property.
<li>The element is a replaced element with an intrinsic height
and <span class="propinst-height">'height'</span> has the value 'auto'.
<li>The height is given by the different between the
<span class="propinst-top">'top'</span> and
<span class="propinst-bottom">'bottom'</span> offsets
(minus padding, borders, and margins). When
padding, border, and margin properties have the specified
value 'auto', the computed value is '0'.
In this case, the height of the box's containing
block must be known, otherwise the <span
class="propinst-bottom">'bottom'</span> of the
box cannot be specified and the height is undefined.
</ol>

<P>In cases one and two, values of <span
class="propinst-top">'top'</span> and <span
class="propinst-bottom">'bottom'</span> are computed as follows:</p>

<ul>
<li>If both <span class="propinst-top">'top'</span> and
<span class="propinst-bottom">'bottom'</span> have the value
'auto', <span class="propinst-top">'top'</span> is the
offset between the top containing block edge and
the box's top side as though it were in normal flow. Then <span
class="propinst-bottom">'bottom'</span> is computed based
on the known top offset and height.
<li>If one property has a value other than 'auto',
this known offset and the height are used to calculate
the offset of the opposite side.
<li>If both properties have a value other than 'auto', <span
class="propinst-bottom">'bottom'</span> is treated as though it
were 'auto'.
</ul>

<P>In case three, the values of <span
class="propinst-top">'top'</span>, <span
class="propinst-bottom">'bottom'</span>, and <span
class="propinst-height">'height'</span> are computed as follows:</P>

<ul>
<li>If <span class="propinst-bottom">'bottom'</span> has
the value 'auto', it is assigned the value '0'.
The height is the (absolute) distance
between the bottom containing
block edge and the box's top edge. A
value of 'auto' for <span class="propinst-top">'top'</span>
means the top offset is calculated as though the box
were in normal flow.
<li>For other values of <span class="propinst-bottom">'bottom'</span>,
the width is the (absolute) distance
between the bottom offset
and the position of the box's top edge, calculated as before.
</ul>

<H3><a name="min-max-heights">Minimum and maximum heights</a>: <span
class="propinst-min-height">'min-height'</span> and <span
class="propinst-max-height">'max-height'</span></h3>

<P>It is sometimes useful to constrain the height of elements to a
certain range. Two properties offer this functionality:

<!-- #include src=properties/min-height.srb -->
<!-- #include src=properties/max-height.srb -->

<P>These two properties allow authors to constrain box heights to a
certain range. Values have the following meanings:</p>

<dl>
<dt><span class="index-inst" title="&lt;length&gt;"><span class="value-inst-length"><strong>&lt;length&gt;</strong></span></span>
<dd>Specifies a fixed minimum or maximum computed height.
<dt><span class="index-inst" title="&lt;percentage&gt;"><span class="value-inst-percentage"><strong>&lt;percentage&gt;</strong></span></span>
<dd>Specifies a percentage for determining the computed value.
The percentage is calculated
with respect
to the generated box's <a href="visuren.html#containing-block">containing block</a>.
</dl>

<P>The following algorithm describes how the two properties influence
the <a href="cascade.html#computed-value">computed value</a>
of the <span class="propinst-height">'height'</span> property:</p>

<OL>

<LI>the <a href="#normal-height">normal height</a> is computed
(without <span class="propinst-min-height">'min-height'</span> and
<span class="propinst-max-height">'max-height'</span>).

<LI>if the value of <span
class="propinst-min-height">'min-height'</span> is greater than the
value of <span class="propinst-max-height">'max-height'</span>, <span
class="propinst-max-height">'max-height'</span> is set to the
value of <span class="propinst-min-height">'min-height'</span>

<LI>if the calculated height is greater than <span
class="propinst-max-height">'max-height'</span>, the value of <span
class="propinst-height">'height'</span> is set to <span
class="propinst-max-height">'max-height'</span>.

<LI>if the calculated height is smaller than <span
class="propinst-min-height">'min-height'</span>, the value of <span
class="propinst-height">'height'</span> is set to <span
class="propinst-min-height">'min-height'</span>.

</OL>


<H2><a name="line-height">Line height calculations</a>: the <span
class="propinst-line-height">'line-height'</span> and <span
class="propinst-vertical-align">'vertical-align'</span>
properties</H2>

<P>As described in the section on <a
href="visuren.html#inline-formatting">inline formatting contexts</a>,
user agents flow inline boxes into a vertical stack of <a
href="visuren.html#line-box">line boxes</a>. The height of a line box
is determined as follows:</p>

<ol>
<li>The height of each inline box in the line box is calculated.  The
height of an inline box is given by its <span
class="propinst-line-height">'line-height'</span> property, unless it
is generated by
a <a href="conform.html#replaced-element">replaced element</a>, in
gives the height of the inline box.

<li>The inline boxes are aligned vertically according
to their <span class="propinst-vertical-align">'vertical-align'</span>
property. If an element
has the values 'top' or 'bottom' for this property, only
the height of the generated boxes affects the line box
height calculation; the boxes cannot be aligned until the
line box has been fully constructed.

<li>The line box height is the distance between the uppermost
box top and the lowermost box bottom.
</ol>

<p>Empty inline elements generate empty inline boxes, but these boxes
still have margins, padding, borders and a line height, and thus
influence these calculations just like elements with content.

<P>Note that if all the boxes in the line box are aligned along their
bottoms, the line box will be exactly the height of the tallest box.
If, however, the boxes are aligned along a common baseline, the line
box top and bottom may not touch the top and bottom of the tallest
box.

<P>Note that top and bottom margins, borders, and padding specified
for <a href="visuren.html#inline-level">inline-level</a> elements do
<em>not</em> enter into the calculation of line box heights
but are visible (inside and outside line
boxes).

<H3>Leading and half-leading</h3>

<P>Since the height of an inline box may be different from the font size
of text in the box (e.g., <span
class="propinst-line-height">'line-height'</span> &gt; 1em), there may
be space above and below rendered glyphs.  The difference between the
font size and the (actual) value of <span
class="propinst-line-height">'line-height'</span> is called the <span
class="index-def" title="leading"><EM>leading</EM></span>. Half the
leading is called the <span class="index-def"
title="half-leading"><EM>half-leading</EM></span>.

<P>User agents center the glyph vertically in the box, adding
half-leading on the top and bottom.  For example, if a piece of text
is '12pt' high and the <span
class="propinst-line-height">'line-height'</span> value is '14pt',
2pts of extra space should be added: 1pt above and 1pt below the
letters. (This applies to empty boxes as well, as if the empty box
contained an infinitely narrow letter.)

<P>When the <span class="propinst-line-height">'line-height'</span>
value is less than the font size, the final line box height will be
less than the font size and the rendered glyphs will "bleed" outside
the box. If such a box touches the edge of a line box, the rendered
glyphs will also "bleed" into the adjacent line box.

<P>Although margins, borders, and padding do not enter into the line
box height calculation, they are still rendered around inline boxes
(except where boxes are split across lines).  This means that if the
height of a line box is shorter than the <a
href="box.html#outer-edge">outer edges</a> of the boxes it
contains, backgrounds and colors may "bleed" into adjacent line
boxes. However, in this case, some user agents may use the line box to
"clip" the border and padding areas (i.e., not render them).

<!-- #include src=properties/line-height.srb -->

<p>If the property is set on a <a
href="visuren.html#block-level">block-level</a> element whose content
is composed of <a href="visuren.html#inline-level">inline-level</a>
elements, it specifies the <em>minimal</em> height of each inline box.

<p>If the property is set on an <a
href="visuren.html#inline-level">inline-level</a> element, it
specifies the <em>exact</em> height of each box generated by the
element. (Except for inline <a
href="conform.html#replaced-element">replaced elements</a>, where the
height of the box is given by the <span
class="propinst-height">'height'</span> property.)

<P>Values for this property have the following meanings:</p>

<dl>
<dt><strong>normal</strong>
<dd>Tells user agents to set the <a
href="cascade.html#computed-value">
computed value</a> to a "reasonable"
value based on the font size of the element. The
value has the same meaning as <span class="index-inst" title="&lt;number&gt;"><span class="value-inst-number">&lt;number&gt;</span></span>
We recommend a number between 1.0 to 1.2.

<dt><span class="index-inst" title="&lt;length&gt;"><span
class="value-inst-length"><strong>&lt;length&gt;</strong></span></span>
<dd>The box height is set to this length. Negative values are illegal.

<dt><span class="index-inst" title="&lt;number&gt;"><span
class="value-inst-number"><strong>&lt;number&gt;</strong></span></span>
<dd>The
<a href="cascade.html#computed-value">
computed value</a> of the property is this number multiplied by the
element's font size. Negative values are illegal.  However, the
number, not the <a href="cascade.html#actual-value">actual value</a>,
is inherited.

<dt><span class="index-inst" title="&lt;percentage&gt;"><span
class="value-inst-percentage"><strong>&lt;percentage&gt;</strong></span></span>
<dd>The <a href="cascade.html#actual-value"> actual value</a> of the
property is this percentage multiplied by the element's font size.
Negative values are illegal.

</dl>

<P>Note that replaced elements have a <span
class="propinst-font-size">'font-size'</span> and a <span
class="propinst-line-height">'line-height'</span> property, even if
they are not used directly to determine the height of the box. The
<span class="propinst-font-size">'font-size'</span> is, however, used
to define the 'em' and 'ex' units, and the 'line-height' has a role in
the <span class="propinst-vertical-align">'vertical-align'</span>
property.

<div class="example"><P>
The three rules in the example below have the same resultant line height:

<PRE>
DIV { line-height: 1.2; font-size: 10pt }     /* number */
DIV { line-height: 1.2em; font-size: 10pt }   /* length */
DIV { line-height: 120%; font-size: 10pt }    /* percentage */
</PRE>
</div>

<P>When an element contains text that is rendered in more than one
font, user agents should determine the <span
class="propinst-line-height">'line-height'</span> value according to
the largest font size. <!-- change this if font-size inheritance is
changed as per HWL's proposal -->

<P>Generally, when there is only one value of <span
class="propinst-line-height">'line-height'</span> for all inline
boxes in a paragraph (and no tall images), the above will ensure that
baselines of successive lines are exactly <span
class="propinst-line-height">'line-height'</span> apart. This is
important when columns of text in different fonts have to be aligned,
for example in a table.

<!-- #include src=properties/vertical-align.srb -->

<P>This property affects the vertical positioning inside a line box of
the boxes generated by an inline-level element. The following values
only have meaning with respect to a parent inline-level element, or to
a parent block-level element, if that element generates <a
href="visuren.html#anonymous">anonymous inline boxes</a>; they have no
effect if no such parent exists</P>
 
<div class="note"><P>
<em><strong>Note.</strong> Values of this property have
slightly different meanings in the context of tables.
Please consult the section on <a href="tables.html#vertical-align">
vertical alignment of table context</a> for details.
</em>
</div>

<DL>
  <DT><strong>baseline</strong>
  <DD>Align the baseline of the box with the baseline of
      the parent box. If the box doesn't have a baseline,
      align the bottom of the box with the parent's baseline.
  <DT><strong>middle</strong>
  <DD>Align the vertical midpoint of the box with the baseline
      of the parent box plus half the x-height of the parent.
  <DT><strong>sub</strong>
  <DD>Lower the baseline of the box to the proper position for
      subscripts of the parent's box. (This value has no effect on the
      font size of the element's text.)
  <DT><strong>super</strong>
  <DD>Raise the baseline of the box to the proper position for
      superscripts of the parent's box. (This value has no effect on the
      font size of the element's text.)
  <DT><strong>text-top</strong>
  <DD>Align the top of the box with the top of the parent element's font.
  <DT><strong>text-bottom</strong>
  <DD>Align the bottom of the box with the bottom of the parent 
      element's font.
  <DT><span class="index-inst" title="&lt;percentage&gt;"><span
  class="value-inst-percentage"><strong>&lt;percentage&gt;
  </strong></span></span>
  <DD>Raise (positive value) or lower (negative value) the box
      by this distance (a percentage of the <span
      class="propinst-line-height">'line-height'</span> value).
      The value '0%' means the same as 'baseline'.
  <DT><span class="index-inst" title="&lt;length&gt;"><span
  class="value-inst-length"><strong>&lt;length&gt;
  </strong></span></span>
  <DD>Raise (positive value) or lower (negative value) the box
      by this distance.
      The value '0cm' means the same as 'baseline'.
</DL>

<P>The remaining values refer to the line box in which the
generated box appears:</p>

<DL>
  <DT><strong>top</strong>
  <DD>Align the top of the box with the top of the line box.
  <DT><strong>bottom</strong>
  <DD>Align the bottom of the box with the bottom of the line box.
</DL>

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