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A3CA
/div>
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<pre class='metadata'>
Title: CSS Fonts Module Level 4
Shortname: css-fonts
Level: 4
Status: ED
Work Status: Exploring
Group: CSSWG
ED: https://drafts.csswg.org/css-fonts-4/
TR: https://www.w3.org/TR/css-fonts-4/
Previous Version: https://www.w3.org/TR/2017/WD-css-fonts-4-20170711/
Editor: John Daggett, Invited Expert, https://twitter.com/nattokirai, w3cid 41498
Editor: Myles C. Maxfield, Apple Inc., mmaxfield@apple.com, w3cid 77180
Editor: Chris Lilley, W3C, http://svgees.us, w3cid 1438
Abstract: This specification defines modifications to the existing <a href="https://drafts.csswg.org/css-fonts-3/">CSS Fonts 3</a> specification along with additional experimental features.
At Risk: Synthesis of the 'font-variant-position' property
At Risk: The 'font-language-override!!property' property
At Risk: The 'font-language-override!!descriptor' descriptor
Ignored Terms: font-palette, <named-palette-color>
</pre>
<pre class="link-defaults">
spec:css-color-4; type:property; text:color
spec:css22; type:value; for:/; text:block
</pre>
<h2 id="introduction">
Introduction</h2>
The CSS3 Fonts specification ([[!CSS-FONTS-3]]) describes the basic
controls CSS provides for selecting and using fonts within documents.
The ideas here are additions or modifications to the properties and
rules defined in CSS3 Fonts.
Issue: Please note that OpenType/TrueType variable font support
is still in intial stages of specification and implementation. Please
comment on any bugs in this spec at <a href="https://github.com/w3c/csswg-drafts/issues">
https://github.com/w3c/csswg-drafts/issues</a>. Also, please note that this
spec is incomplete and only includes some of the text from <a href="https://www.w3.org/TR/css-fonts-3/">
https://www.w3.org/TR/css-fonts-3/</a>.
<h2 id="basic-font-props">
Basic Font Properties</h2>
The particular font face used to render a character is determined by
the font family and other font properties that apply to a given element.
This structure allows settings to be varied independent of each
other.
<h3 id="font-family-prop">
Font family: the 'font-family!!property' property</h3>
<pre class="propdef">
Name: font-family
Value: [ <<family-name>> | <<generic-family>> ] #
Initial: depends on user agent
Applies to: all elements
Inherited: yes
Percentages: n/a
Computed value: list, each item a string and/or <<generic-family>> keywords
Animation type: discrete
</pre>
This property specifies a prioritized list of font family names or generic family names.
A font family defines a set of faces that vary in weight, width or slope.
CSS uses the combination of a family name with other font properties to select an individual face.
Using this selection mechanism,
rather than selecting a face via the style name as is often done in design applications,
allows some degree of regularity in textual display when fallback occurs.
Component values are a comma-separated list indicating alternatives.
A user agent iterates through the list of family names
until it matches an available font
that contains a glyph for the character to be rendered.
(See [[#cluster-matching]].)
This allows for differences in available fonts across platforms and
for differences in the range of characters supported by individual fonts.
<div class="example">
<pre>
body {
font-family: Helvetica, Verdana, sans-serif;
}
</pre>
If Helvetica is available, it will be used when rendering.
If neither Helvetica nor Verdana is present,
then the generic font-family ''sans-serif'' font will be used.
</div>
There are two types of font family names:
<dl>
<dt><dfn id="family-name-value"><<family-name>></dfn>
<dd>
The name of a font family, such as Helvetica or Verdana in the previous example.
<dt><dfn id="generic-family-value"><<generic-family>></dfn>
<dd>
Each <<generic-family>> keyword represents
a generic font choice,
and behaves as an alias for one or more locally-installed fonts
belonging to the specified generic font category.
A <<generic-family>> can thus be used as a reliable fallback
for when an author's more specific font choices are not available.
Authors are encouraged to append a generic font family as a last alternative
for improved robustness.
Note that <<generic-family>> keywords cannot be quoted
(otherwise they are interpreted as a <<family-name>>).
The following generic family keywords are defined:
''serif'', ''sans-serif'', ''cursive'', ''fantasy'', ''monospace'', ''system-ui'', ''emoji'', ''math'' and ''fangsong''.
See [[#generic-font-families]].
</dl>
<h4 id="family-name-syntax">
</h4>
Font family names other than generic families must either be given quoted as <<string>>s,
or unquoted as a sequence of one or more <a>identifiers</a>.
Note: This means most punctuation characters and digits at the start of
each token must be escaped in unquoted font family names.
<div class="example">
To illustrate this, the following declarations are invalid:
<pre>
font-family: Red/Black, sans-serif;
font-family: "Lucida" Grande, sans-serif;
font-family: Ahem!, sans-serif;
font-family: test@foo, sans-serif;
font-family: #POUND, sans-serif;
font-family: Hawaii 5-0, sans-serif;
</pre>
</div>
If a sequence of identifiers is given as a <<family-name>>,
the computed value is the name
converted to a string
by joining all the identifiers in the sequence by single spaces.
To avoid mistakes in escaping,
it is recommended to quote font family names
that contain white space, digits,
or punctuation characters other than hyphens:
<div class="example">
Quoting font families prevents escaping mistakes.
<pre>
body { font-family: "New Century Schoolbook", serif }
<body style="font-family: '21st Century', fantasy">
</pre>
</div>
Font family <em>names</em> that happen to be the same as
a 'font-family!!property' keyword value
(e.g. <a>CSS-wide keywords</a> such as ''inherit'', or
<<generic-family>> keywords such as ''serif'')
must be quoted to prevent confusion
with the keywords of the same names.
UAs must not consider these keywords as matching the <<family-name>> type.
<h4 id="font-families">
Relationship Between Faces and Families
</h4>
A font family name only specifies a name given to a set of font faces;
it does not specify an individual face.
<div class="example">
For example, given the availability of the fonts below,
Futura would match but Futura Medium would not:
<figure>
<img alt="family and face names" src="images/familyvsfacename.png">
<figcaption>Family and individual face names</figcaption>
</figure>
</div>
Note: The CSS definition of font attributes used for selection
are explicitly not intended to define a font taxonomy.
A type designer's idea of a family can often extend
to a set of faces that vary along axes other than just
the standard axes of
weight ('font-weight!!property'),
width ('font-stretch!!property'),
and slant ('font-style!!property').
A family can vary along axes that are unique to that family.
The CSS font selection mechanism merely
provides a way to determine the “closest” match
when substitution is necessary.
Note: The precise way a set of fonts are grouped into font families
varies depending upon the platform font management APIs.
For example, the Windows GDI API only allows four faces to be grouped into a family,
while the DirectWrite API,
Core Text API,
and other platforms support
font families with a variety of weights, widths, and slopes
(see [[#platform-props-to-css]] for more details).
See [[#localized-name-matching]] below for information on how
font-family names are matched.
<h4 id="generic-font-families">
Generic font families</h4>
Each generic font family must always map to at least one matched font face.
However, a single generic font family may be a composite face
combining different typefaces based on such things as
the Unicode range of the character,
the <a>content language</a> of the containing element,
user preferences, system settings, etc.
Different generic font families may map to the same used font.
User agents should provide reasonable default choices for the generic font families,
that express the characteristics of each family as well as possible,
within the limits allowed by the underlying technology.
User agents are encouraged to allow users to select alternative faces for the generic font families.
<dl dfn-for="font-family,<generic-family>" dfn-type=value>
<dt id="serif-def"><dfn>serif</dfn>
<dd>
Serif fonts represent the formal text style for a script.
This often means, but is not limited to,
glyphs that have finishing strokes,
flared or tapering ends,
or have actual serifed endings (including slab serifs).
Serif fonts are typically proportionately-spaced.
They often display a greater variation between thick and thin strokes
than fonts from the ''sans-serif'' generic font family.
CSS uses the term "serif" to apply to a font for any script,
although other names might be more familiar for particular scripts,
such as Mincho (Japanese),
Sung or Song (Chinese),
Batang (Korean).
For Arabic, the Naskh style would correspond to ''serif''.
This is due to its typographic role, rather than its design style.
Any font that fits this typographic role
may be used to represent the generic ''serif'' family.
<figure>
<img alt="sample serif fonts" src="images/serifexamples.png" >
<figcaption>Sample serif fonts</figcaption>
</figure>
<dt id="sans-serif-def"><dfn>sans-serif</dfn>
<dd>
Glyphs in sans-serif fonts,
as the term is used in CSS,
are generally low contrast
(vertical and horizontal stems have the close to the same thickness)
and have stroke endings that are plain
(without any flaring, cross stroke, or other ornamentation).
Sans-serif fonts are typically proportionately-spaced.
They often have little variation between thick and thin strokes,
compared to fonts from the ''serif'' family.
CSS uses the term "sans-serif" to apply to a font for any script,
although other names might be more familiar for particular scripts,
such as Gothic (Japanese),
Hei (Chinese),
or Gulim (Korean).
Any font that fits this typographic role
may be used to represent the generic ''sans-serif'' family.
<figure>
<img alt="sample sans-serif fonts" src="images/sansserifexamples.png" >
<figcaption>Sample sans-serif fonts</figcaption>
</figure>
<dt id="cursive-def"><dfn>cursive</dfn>
<dd>
Glyphs in cursive fonts generally use a more informal script style,
and the result looks more like handwritten pen or brush writing than printed letterwork.
For example, Kaiti (Chinese), which uses a brush-based style,
would be classified as a CSS ''cursive'' font family.
CSS uses the term "cursive" to apply to a font for any script,
although other names such as Chancery, Brush, Swing and Script are also used in font names.
<figure>
<img alt="sample cursive fonts" src="images/cursiveexamples.png" >
<figcaption>Sample cursive fonts</figcaption>
</figure>
<dt id="fantasy-def"><dfn>fantasy</dfn>
<dd>
Fantasy fonts are primarily decorative or expressive fonts
that contain decorative or expressive representations of characters.
These do not include Pi or Picture fonts which do not represent actual characters.
<figure>
<img alt="sample fantasy fonts" src="images/fantasyexamples.png" >
<figcaption>Sample fantasy fonts</figcaption>
</figure>
<dt id="monospace-def"><dfn>monospace</dfn>
<dd>
The sole criterion of a monospace font is that all glyphs have the same fixed width.
This is often used to render samples of computer code.
<figure>
<img alt="sample monospace fonts" src="images/monospaceexamples.png" >
<figcaption>Sample monospace fonts</figcaption>
</figure>
<dt id="system-ui-def"><dfn>system-ui</dfn>
<dd>
This generic font family lets text render with the default user interface font
on the platform on which the UA is running.
A cross-platform UA should use different fonts on its different supported platforms.
The purpose of ''system-ui'' is to allow web content
to integrate with the look and feel of the native OS.
On platforms which have a collection of system user interface fonts
(e.g. for different languages),
user agents may treat ''system-ui'' as a virtual font
which encompasses all the relevant platform user interface fonts.
<div class="example">
As with other generic font families,
the substitution of specific installed fonts for ''system-ui''
does not affect the computed style.
<pre>
<div id="system-text" style="font-family: system-ui"></div>
...
window.getComputedStyle(document.getElementById("system-text")).getPropertyValue("font-family");
</pre>
The script above should not have any knowledge of how ''system-ui''
is expanded to include a collection of system user interface fonts.
In particular, the above script should yield a result of "system-ui" on every platform.
</div>
<dt id="emoji-def"><dfn>emoji</dfn>
<dd>
This font family is intended for use with emoji characters.
Issue: Add more description
<dt id="math-def"><dfn>math</dfn>
<dd>
This font family is intended for use with mathematical expressions.
Issue: Add more description
<dt id="fangsong-def"><dfn>fangsong</dfn>
<dd>
This font family is used for Fang Song (仿宋) typefaces in Chinese.
Fang Song is a relaxed, indermediate form
between Song (''serif'') and Kai (''cursive'').
Typically, the horizontal lines are tilted,
the endpoint flourishes are smaller,
and there is less variation in stroke width,
compared to a Song style.
Fang Song is often used for official Chinese Government documents.
</dl>
<h3 id="font-weight-prop">Font weight: the 'font-weight!!property' property</h3>
<pre class="propdef">
Name: font-weight
Value: <<font-weight-absolute>> | bolder | lighter
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: n/a
Computed value: a number, see below
Animation type: by computed value type
</pre>
The 'font-weight!!property' property specifies the weight of glyphs in the font,
their degree of blackness, or stroke thickness.
This property accepts values of the following:
<pre class="prod"><dfn id="font-weight-absolute-values"><font-weight-absolute></dfn> = [normal | bold | <<number>>]</pre>
Values have the following meanings:
<dl dfn-for=font-weight dfn-type=value>
<dt id="font-weight-numeric-values"><dfn><<number>></dfn>
<dd>
Each number indicates a weight that is at least as dark as its predecessor.
Only values greater than or equal to 1, and less than or equal to 1000, are valid,
and all other values are <a>invalid</a>.
Numeric values typically correspond to the commonly used weight names below.
<ul>
<li>100 - Thin
<li>200 - Extra Light (Ultra Light)
<li>300 - Light
<li>400 - Normal
<li>500 - Medium
<li>600 - Semi Bold (Demi Bold)
<li>700 - Bold
<li>800 - Extra Bold (Ultra Bold)
<li>900 - Black (Heavy)
</ul>
Note: A font might internally provide its own weight name mappings,
but those mappings within the font are disregarded in CSS.
<dt><dfn>normal</dfn>
<dd>
Same as ''400''.
<dt><dfn>bold</dfn>
<dd>
Same as ''700''.
<dt><dfn>bolder</dfn>
<dd>
Specifies a bolder weight than the inherited value.
See [[#relative-weights]].
<dt><dfn>lighter</dfn>
<dd>
Specifies a lighter weight than the inherited value.
See [[#relative-weights]].
</dl>
Font formats that use a scale other than a nine-step scale
should map their scale onto the CSS scale
so that 400 roughly corresponds with a face that would be labeled as Regular, Book, Roman
and 700 roughly matches a face that would be labeled as Bold.
Alternately, weights may be inferred from style names
that correspond roughly with the scale above.
The scale is relative,
so a face with a larger weight value must never appear lighter.
If style names are used to infer weights,
care should be taken to handle variations in style names across locales.
<h4 id="relative-weights">
Relative Weights</h4>
Specified values of ''bolder'' and ''lighter'' indicate weights
relative to the weight of the parent element.
The computed weight is calculated based on the inherited 'font-weight!!property' value
using the chart below.
<table id="bolderlighter" class="data" summary="Bolder/lighter mappings">
<thead>
<tr>
<th>Inherited value (<var>w</var>)
<th>bolder
<th>lighter
<tbody>
<tr><th><var>w</var> < 100<td>400<td>No change
<tr><th>100 ≤ <var>w</var> < 350<td>400<td>100
<tr><th>350 ≤ <var>w</var> < 550<td>700<td>100
<tr><th>550 ≤ <var>w</var> < 750<td>900<td>400
<tr><th>750 ≤ <var>w</var> < 900<td>900<td>700
<tr><th>900 ≤ <var>w</var><td>No change<td>700
</table>
Note: The table above is equivalent to selecting the next relative bolder or lighter face,
given a font family containing normal and bold faces
along with a thin and a heavy face.
Authors who desire finer control over the exact weight values used for a given element
can use numerical values instead of relative weights.
<h4 id="missing-weights">
Missing weights</h4>
Quite often there are only a few weights available for a particular font family.
When a weight is specified for which no face exists, a face with a nearby weight is used.
In general, bold weights map to faces with heavier weights and light weights map to faces with lighter weights.
(See the [[#font-matching-algorithm]] for a precise definition.)
<div class="example">
The examples here illustrate which face is used for different weights.
Grey indicates that a face for the desired weight does not exist, so a face with a nearby weight is used.
<figure>
<img alt="weight mappings for a family with 400, 700 and 900 weights" src="images/optimaweights.png" >
<figcaption>Weight mappings for a font family with 400, 700 and 900 weight faces</figcaption>
</figure>
<figure>
<img alt="weight mappings for a family with 300, 600 weights" src="images/hiraginoweights.png" >
<figcaption>Weight mappings for a font family with 300 and 600 weight faces</figcaption>
</figure>
</div>
Most user agents model a font as having a particular weight
which often corresponds to one of the numbers in the nine-step scale
described <a href="#font-weight-numeric-values">above</a>.
While this is true of most fonts, some fonts might be configurable so as to support a range of weights.
In this situation, the user agent uses a face with a weight as close as possible to the weight requested
(see [[#font-matching-algorithm]] for the precise algorithm).
In particular, a user agent using a font which supports a range of weights
should behave the same as if a font is present at each individual weight in the range.
For TrueType / OpenType fonts that use variations,
the <code>wght</code> variation is used to implement varying weights.
Fractional weights are valid.
Although the practice is not well-loved by typographers,
bold faces are often synthesized by user agents for families that lack actual bold faces.
For the purposes of font matching,
these faces must be treated as if they exist within the family.
Authors can explicitly avoid this behavior by using the 'font-synthesis' property.
<h3 id="font-stretch-prop">
Font width: the 'font-stretch!!property' property</h3>
<pre class="propdef">
Name: font-stretch
Value: normal | <<percentage>> | ultra-condensed | extra-condensed | condensed | semi-condensed | semi-expanded | expanded | extra-expanded | ultra-expanded
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: Not resolved
Computed value: a percentage, see below
Animation type: by computed value type
</pre>
The 'font-stretch!!property' property selects a normal,
condensed, or expanded face from a font family.
Values are specified either as percentages
or as keywords which map to a percentage
as defined in the following table:
<table id="stretchmappings" class="data">
<thead>
<tr>
<th>Absolute keyword value
<th>Numeric value
<tbody dfn-for=font-stretch dfn-type=value>
<tr><th><dfn>ultra-condensed</dfn> <td>50%
<tr><th><dfn>extra-condensed</dfn> <td>62.5%
<tr><th><dfn>condensed</dfn> <td>75%
<tr><th><dfn>semi-condensed</dfn> <td>87.5%
<tr><th><dfn>normal</dfn> <td>100%
<tr><th><dfn>semi-expanded</dfn> <td>112.5%
<tr><th><dfn>expanded</dfn> <td>125%
<tr><th><dfn>extra-expanded</dfn> <td>150%
<tr><th><dfn>ultra-expanded</dfn> <td>200%
</table>
<dfn value for=font-stretch><<percentage>></dfn>
values represent the fractional width of the glyphs,
with 100% representing “normal” glyph widths
(as defined by the font designer).
Values less than 0% are <a>invalid</a>.
When a face does not exist for a given width,
values less than 100% map to a narrower face if one exists, otherwise a wider face.
Conversely, values greater than or equal to 100% map to a wider face if one exists, otherwise a narrower face.
Some fonts might support a range of stretch values;
if the requested stretch value is not available in the font,
the closest supported value is used, using the same mapping rules
(see the [[#font-matching-algorithm]] for the precise algorithm).
For TrueType / OpenType fonts that support variations,
the <code>wdth</code> variation is used to implement varying widths.
<div class="example">
The figure below shows how nine font-stretch property settings
affect font matching for a font family containing a variety of discrete widths.
Grey indicates a width for which no face exists and a different width is substituted:
<figure>
<img alt="width mappings for a family with condensed, normal and expanded faces" src="images/universwidths.png" >
<figcaption>Width mappings for a font family with condensed, normal and expanded width faces</figcaption>
</figure>
</div>
User Agents must not sythesize stretched faces for font families which lack actual stretched faces.
For compatibility with [[CSS-FONTS-3]],
{{getComputedStyle()}} serializes values
that correspond to one of the 'font-stretch!!property' keywords
as that keyword
(instead of as a <<percentage>>).
<h3 id="font-style-prop">
Font style: the 'font-style!!property' property</h3>
<pre class="propdef">
Name: font-style
Value: normal | italic | oblique <<angle>>?
Initial: normal
Applies to: all elements
Inherited: yes
Percentages: n/a
Computed value: the keyword specified, pl
2334
us angle in degrees if specified
Animation type: by computed value type
</pre>
The 'font-style!!property' property allows italic or oblique faces to be selected.
Italic forms are generally cursive in nature while oblique faces are typically sloped versions of the regular face.
<div class="example">
Compare the artificially sloped renderings of Palatino "a" and Baskerville "N" in grey
with the actual italic versions:
<figure>
<img alt="artificial sloping vs. real italics" src="images/realvsfakeitalics.png" >
<figcaption>Artificial sloping versus real italics</figcaption>
</figure>
</div>
Values have the following meanings:
<dl dfn-for=font-style dfn-type=value>
<dt><dfn>normal</dfn>
<dd>
Matches against a face that is classified as a normal face,
one that is neither italic or obliqued.
This represents an oblique value of "0".
<dt><dfn>italic</dfn>
<dd>
Matches against a font that is labeled as an italic face,
or an oblique face if one does not exist.
<dt><dfn>oblique <<angle>>?</dfn>
<dd>
Controls matching against an oblique face.
Positive angles represent a clockwise slant;
negative angles represent a counter-clockwise slant.
The lack of an <<angle>> represents ''14deg''.
(Note that a font might internally provide its own mapping for "oblique",
but the mapping within the font is disregarded.)
Fractional and negative values are accepted; however,
values less than -90deg or values greater than 90deg are <a>invalid</a>.
If no oblique faces exist, this value can match an italic face.
</dl>
A font family might contain no italic or oblique faces,
only an italic face and no oblique,
only an oblique face and no italic,
both an oblique and an italic,
multiple oblique faces at various angles,
or various combinations thereof.
The font matching routine will select a font to use which is closest to the requested angle.
In general, for a requested angle greater or equal to 20deg,
larger angles are prefered;
otherwise, smaller angles are preferred.
(See [[#font-matching-algorithm]].)
For TrueType / OpenType fonts that use variations,
the <code>slnt</code> variation is used to implement oblique values,
and the <code>ital</code> variation with a value of 1 is used to implement the italic values.
ISSUE: The threshold for preferring oblique over normal
<a href="https://github.com/w3c/csswg-drafts/issues/2295">should be lower than the average angle</a>.
Issue: What direction should positive and negative obliques skew in vertical writing mode?
How do we achieve skews in the opposite dimension
(needed for vertical writing)?
If no italic or oblique face is available,
oblique faces may be synthesized by rendering non-obliqued faces
with an artificial obliquing operation.
The use of these artificially obliqued faces
can be disabled using the 'font-synthesis' property.
<!-- resolution on the "undefined" nature of synthetics: https://www.w3.org/2013/06/06-css-minutes.html#item03 -->
Note: While oblique faces can be simulated by artificially sloping the glyphs of the regular face,
this is not equivalent to a true oblique,
in which optical stroke thicknesses are properly preserved despite the slant.
It is always better to use an actual oblique font rather than rely on a synthetic version.
For the purposes of font matching,
User Agents may treat ''italic'' as a synonym for ''oblique''.
For User Agents that treat these values distinctly,
synthesis must not be performed for ''italic''.
Note: Authors should also be aware that synthesized approaches might not be suitable
for scripts like Cyrillic, where italic forms are very different in shape.
It is always better to use an actual italic font rather than rely on a synthetic version.
Note: Many scripts lack the tradition of mixing a cursive form within text rendered with a normal face.
Chinese, Japanese and Korean fonts almost always lack italic or oblique faces.
Fonts that support a mixture of scripts
will sometimes omit specific scripts, such as Arabic,
from the set of glyphs supported in the italic face.
User agents should be careful about making <a>character map</a> assumptions across faces
when implementing synthesis across fonts,
as italic faces in a family can have different <a>character maps</a> than Roman faces.
<h3 id="font-size-prop">
Font size: the 'font-size' property</h3>
<pre class="propdef">
Name: font-size
Value: <<absolute-size>> | <<relative-size>> | <<length-percentage>>
Initial: medium
Applies to: all elements
Inherited: yes
Percentages: refer to parent element's font size
Computed value: an absolute length, as clamped by 'font-min-size' and 'font-max-size'
Animation type: by computed value type`
</pre>
This property indicates the desired height of glyphs from the font.
For scalable fonts, the font-size is a scale factor applied to the EM unit of the font.
For non-scalable fonts, the font-size is converted into absolute units
and matched against the declared font-size of the font,
using the same absolute coordinate space for both of the matched values.
Note: There is no requirement on how closely a glyph should fit its EM box.
Individual fonts can have a different apparent visual size
when rendered at the same 'font-size'.
Additionally, glyphs might render arbitrarily far outside their EM box;
and if they overflow their <a>containing block</a>
can induce <a>ink overflow</a>.
Values have the following meanings:
<dl dfn-for=font-size dfn-type=value>
<dt><dfn><<absolute-size>></dfn>
<dd>
An <<absolute-size>> keyword refers to an entry in a table of font sizes
computed and kept by the user agent.
See [[#absolute-size-mapping]].
Possible values are:
<pre class=prod>[ xx-small | x-small | small | medium | large | x-large | xx-large ]</pre>
<dt><dfn><<relative-size>></dfn>
<dd>
A <<relative-size>> keyword is interpreted
relative to the computed 'font-size' of the parent element
and possibly the table of font sizes. Possible values are:
<pre class=prod>[ larger | smaller ]</pre>
If the parent element has a keyword font size in the absolute size keyword mapping table,
''larger'' may compute the font size to the next entry in the table,
and ''smaller'' may compute the font size to the previous entry in the table.
For example, if the parent element has a font size of ''medium'',
specifying a value of ''larger'' may make the font size of the child element ''large''.
Instead of using next and previous items in the previous keyword table,
User Agents may instead use a simple ratio
to increase or decrease the font size
relative to the parent element.
The specific ratio is unspecified,
but should be around 1.2–1.5.
This ratio may vary across different elements.
Note: A sight-impared user may request a User Agent use a higher ratio than default,
in order to aid readability.
In addition, a User Agent may choose to use different ratios
when it detects paragraph text as opposed to title text.
<dt><dfn><<length-percentage>></dfn>
<dd>
A length value specifies an absolute font size
(independent of the user agent's font table).
Negative lengths are invalid.
A percentage value specifies an absolute font size relative
to the parent element's computed 'font-size'.
Negative percentages are invalid.
Note: Use of percentage values or <a>font-relative lengths</a>
such as ''em''s and ''rem''s
leads to more robust and cascadable style sheets.
</dl>
<div class="example">
The following style sheet demonstrates
various ways of specifying the font size.
<pre>
p { font-size: 12pt; }
blockquote { font-size: larger }
em { font-size: 150% }
em { font-size: 1.5em }
</pre>
</div>
Note: The computed value of this property is clamped by
the computed values of 'font-min-size' and 'font-max-size'.
Furthermore,
the used value of this property can differ from its computed value
due to 'font-size-adjust'.
However, child elements inherit the computed 'font-size' value,
which is not affected by 'font-size-adjust'
(otherwise, the effect of 'font-size-adjust' would compound).
The actual value of this property can differ from the used value
due to the unavailability of certain font sizes.
<h4 id="absolute-size-mapping">
Absolute Size Keyword Mapping Table</h4>
The following table provides user agent guidelines for the absolute-size scaling factor
and their mapping to HTML heading and absolute font-sizes.
The ''font-size/medium'' value is used as the reference middle value.
The user agent may fine-tune these values for different fonts or different types of display devices.
<table class="data">
<thead>
<th>CSS absolute-size values
<th>xx-small
<th>x-small
<th>small
<th>medium
<th>large
<th>x-large
<th>xx-large
<th>
<tbody>
<tr>
<th>scaling factor
<td>3/5
<td>3/4
<td>8/9
<td>1
<td>6/5
<td>3/2
<td>2/1
<td>3/1
<tr>
<th>HTML headings
<td>h6
<td>
<td>h5
<td>h4
<td>h3
<td>h2
<td>h1
<td>
<tr>
<th>HTML <{font}> sizes
<td>1
<td>
<td>2
<td>3
<td>4
<td>5
<td>6
<td>7
</table>
Note: In CSS1, the suggested scaling factor between adjacent indexes was 1.5,
which user experience proved to be too large.
In CSS2, the suggested scaling factor for computer screen between adjacent indexes was 1.2
which still created issues for the small sizes.
The new scaling factor varies between each index to provide a better readability.
To preserve readability, an UA applying these guidelines
should nevertheless avoid creating font sizes
of less than 9 device pixels per EM unit.
<h3 id="font-min-max-size-prop">Minimum and maximum font size: the 'font-min-size' and 'font-max-size' properties</h3>
<pre class="propdef">
Name: font-min-size
Value: <<absolute-size>> | <<relative-size>> | <<length-percentage>>
Initial: 0
Applies to: all elements
Inherited: yes
Percentages: refer to parent element's font size
Computed value: an absolute length
Animation type: by computed value type
</pre>
<pre class="propdef">
Name: font-max-size
Value: <<absolute-size>> | <<relative-size>> | <<length-percentage>> | infinity
Initial: infinity
Applies to: all elements
Inherited: yes
Percentages: refer to parent element's font size
Computed value: an absolute length
Animation type: by computed value type
</pre>
These two properties allow an author or user to require that an element's font size be clamped
within the specified range.
If the specified value 'font-size' is outside the bounds created by
the used 'font-min-size' and 'font-max-size',
the computed value of 'font-size' is clamped to the values specified in these two properties.
If the computed 'font-min-size' is larger than the computed 'font-max-size',
then the used 'font-max-size' is increased to match the 'font-min-size'.
Some user agents provide a non-standard mapping between the computed value of 'font-size' and the used value of 'font-size'.
The interaction of those nonstandard algorithms with 'font-min-size' or 'font-max-size' is explicitly undefined.
Note: Users with accessibility preferences can set these properties
as ''!important'' rules in a <a>user style sheet</a>
to force websites‘ text to become larger or smaller.
<h3 id="font-size-adjust-prop">
Relative sizing: the 'font-size-adjust' property</h3>
<pre class="propdef">
Name: font-size-adjust
Value: none | <<number>>
Initial: none
Applies to: all elements
Inherited: yes
Percentages: N/A
Computed value: a number or the keyword ''font-size-adjust/none''
Animation type: by computed value type
</pre>
For any given font size, the apparent size and effective legibility of text
varies across fonts.
For scripts such as Latin or Cyrillic that
distinguish between upper and lowercase letters,
the relative height of lowercase letters
compared to their uppercase counterparts
is a determining factor of legibility.
This is commonly referred to as the <dfn>aspect value</dfn>
and is equal to the x-height of a font
divided by the font size.
In situations where font fallback occurs,
fallback fonts might not share the same aspect value as the desired font family
and will thus be less readable.
The 'font-size-adjust' property is a way
to preserve the readability of text when font fallback occurs.
It does this by adjusting the font-size
so that the x-height is the same
regardless of the font used.
<div class="example">
The style defined below defines Verdana as the desired font family,
but if Verdana is not available Futura or Times will be used.
<pre>
p {
font-family: Verdana, Futura, Times;
}
<p>Lorem ipsum dolor sit amet, ...</p>
</pre>
Verdana has a relatively high aspect value,
lowercase letters are relatively tall compared to uppercase letters,
so at small sizes text appears legible.
Times has a lower aspect value and so if fallback occurs,
the text will be less legible at small sizes than Verdana.
</div>
How text rendered in each of these fonts compares is shown below,
the columns show text rendered in Verdana, Futura and Times.
The same font-size value is used across cells
within each row and red lines are included to show the differences in x-height.
In the upper half each row is rendered in the same font-size value.
The same is true for the lower half
but in this half the 'font-size-adjust' property is also set
so that the actual font size is adjusted
to preserve the x-height across each row.
Note how small text remains relatively legible across each row in the lower half.
<figure>
<img alt="text with and without 'font-size-adjust'" src="images/fontsizeadjust.png" >
<figcaption>Text with and without the use of 'font-size-adjust'</figcaption>
</figure>
This property allows authors to specify an <a>aspect value</a> for an element
that will effectively preserve the x-height of the first choice font,
whether it is substituted or not.
Values have the following meanings:
<dl dfn-type=value dfn-for=font-size-adjust>
<dt><dfn id="font-size-adjust-none-value">none</dfn>
<dd>
Do not preserve the font's x-height.
<dt><dfn id="aspect-ratio-value"><<number>></dfn>
<dd>
Specifies the <a>aspect value</a> used in the calculation below
to calculate the adjusted font size:
<pre>c = ( a / a' ) s</pre>
where:
<pre>
s = font-size value
a = <a>aspect value</a> as specified by the 'font-size-adjust' property
a' = <a>aspect value</a> of actual font
c = adjusted font-size to use
</pre>
Negative values are invalid.
This value applies to any font that is selected
but in typical usage it should be based on the <a>aspect value</a>
of the first font in the font-family list.
If this is specified accurately,
the <code>(a/a')</code> term in the formula above
is effectively 1 for the first font
and no adjustment occurs.