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Example(s):

For example, text that contains English words mixed with mathematical symbols may need a font set of two fonts, one containing Latin letters and digits, the other containing mathematical symbols. Here is an example of a font set suitable for a text that is expected to contain text with Latin characters, Japanese characters, and mathematical symbols:

BODY { font-family: Baskerville, "Heisi Mincho W3", Symbol, serif }

The glyphs available in the "Baskerville" font (a font that covers only Latin characters) will be taken from that font, Japanese glyphs will be taken from "Heisi Mincho W3", and the mathematical symbol glyphs will come from "Symbol". Any others will come from the generic font family 'serif'.

Example(s):

The richer selector syntax of CSS2 may be used to create language-sensitive typography. For example, some Chinese and Japanese characters are unified to have the same Unicode codepoint, although the abstract glyphs are not the same in the two languages.

 
*:lang(ja-jp) { font: 900 14pt/16pt "Heisei Mincho W9", serif }
*:lang(zh-tw) { font: 800 14pt/16.5pt "Li Sung", serif }

This selects any element that has the given language - Japanese or Traditional Chinese - and requests the appropriate font.

Example(s):

In this example, normal text in an H1, H2, or H3 element will be displayed with an italic font. However, emphasized text (EM) within an H1 will appear in a normal face.

H1, H2, H3 { font-style: italic }
H1 EM { font-style: normal }

Example(s):

The following example results in an H3 element in small-caps, with emphasized words (EM) in oblique small-caps:

H3 { font-variant: small-caps }
EM { font-style: oblique }

Example(s):

P { font-weight: normal }   /* 400 */
H1 { font-weight: 700 }     /* bold */
BODY { font-weight: 400 }
STRONG { font-weight: bolder } /* 500 if available */

Example(s):

P { font-size: 12pt; }
BLOCKQUOTE { font-size: larger }
EM { font-size: 150% }
EM { font-size: 1.5em }

Example(s):

For example, if 14px Verdana (with an aspect value of 0.58) was unavailable and an available font had an aspect value of 0.46, the font-size of the substitute would be 14 * (0.58/0.46) = 17.65px.

Example(s):

P { font: 12pt/14pt sans-serif }
P { font: 80% sans-serif }
P { font: x-large/110% "new century schoolbook", serif }
P { font: bold italic large Palatino, serif }
P { font: normal small-caps 120%/120% fantasy }
P { font: oblique 12pt "Helvetica Nue", serif; font-stretch: condensed }

In the second rule, the font size percentage value ('80%') refers to the font size of the parent element. In the third rule, the line height percentage ('110%') refers to the font size of the element itself.

The first three rules do not specify the 'font-variant' and 'font-weight' explicitly, so these properties receive their initial values ('normal'). Notice that the font family name "new century schoolbook", which contains spaces, is enclosed in quotes. The fourth rule sets the 'font-weight' to 'bold', the 'font-style' to 'italic', and implicitly sets 'font-variant' to 'normal'.

The fifth rule sets the 'font-variant' ('small-caps'), the 'font-size' (120% of the parent's font size), the 'line-height' (120% of the font size) and the 'font-family' ('fantasy'). It follows that the keyword 'normal' applies to the two remaining properties: 'font-style' and 'font-weight'.

The sixth rule sets the 'font-style', 'font-size', and 'font-family', the other font properties being set to their initial values. It then sets 'font-stretch' to 'condensed' since that property cannot be set to that value using the 'font' shorthand property.

Example(s):

 
BUTTON { font: 300 italic 1.3em/1.7em "FB Armada", sans-serif }
BUTTON P { font: menu }
BUTTON P EM { font-weight: bolder }

If the font used for dropdown menus on a particular system happened to be, for example, 9-point Charcoal, with a weight of 600, then P elements that were descendants of BUTTON would be displayed as if this rule were in effect:

 
BUTTON P { font: 600 9pt Charcoal }

Because the 'font' shorthand resets to its initial value any property not explicitly given a value, this has the same effect as this declaration:

 
BUTTON P {
  font-style: normal;
  font-variant: normal;
  font-weight: 600;
  font-size: 9pt;
  line-height: normal;
  font-family: Charcoal
} 
</PRE>
 
</div>
 
<h4>15.2.6 <a name="generic-font-families">Generic font families</a></h4>
 
<p>Generic font families are a fallback mechanism, a means of
preserving some of the style sheet author's intent in the worst case
when none of the specified fonts can be selected. For optimum
typographic control, particular named fonts should be used in
style sheets.
 
 
<p><a name="defined-to-exist">All five generic font families are defined to exist</a> in all
CSS implementations (they need not necessarily map to five distinct
actual fonts). User agents should provide reasonable
default choices for the generic font families, which express the
characteristics of each family as well as possible within the limits
allowed by the underlying technology.
 
<p>User agents are encouraged to allow users to select alternative
choices for the generic fonts.
 
<h5> <span class="index-def" title="serif, definition of"><a name="serif-def"><dfn>serif</dfn></a></span></h5>
 
<p>Glyphs of serif fonts, as the term is used in CSS, 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 may be more familiar for particular scripts, such
as Mincho (Japanese), Sung or Song (Chinese), Totum or Kodig (Korean).
Any font that is so described may be used to represent the
generic 'serif' family.
 
<p>Examples of fonts that fit this description include:</p>
 
<table>
<tr><td>Latin fonts
  <td>Times New Roman, Bodoni,
Garamond, Minion Web, ITC Stone Serif, MS Georgia, Bitstream Cyberbit
<tr><td>Greek fonts
  <td>Bitstream Cyberbit
<tr><td>Cyrillic fonts
  <td>Adobe Minion Cyrillic, Excelcior Cyrillic Upright,
Monotype Albion 70, Bitstream Cyberbit, ER Bukinst
<tr><td>Hebrew fonts
  <td>New Peninim, Raanana, Bitstream Cyberbit
<tr><td>Japanese fonts
  <td>Ryumin Light-KL, Kyokasho ICA, Futo Min A101
<tr><td>Arabic fonts
  <td>Bitstream Cyberbit
<tr><td>Cherokee fonts
  <td>Lo Cicero Cherokee
</table>
 
<h5> <span class="index-def" title="sans-serif, definition of">
<a name="sans-serif-def"><dfn>sans-serif</dfn></a></span></h5>
 
<p>Glyphs in sans-serif fonts, as the term is used in CSS, 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 may be more familiar for particular scripts, such as
Gothic (Japanese), Kai (Chinese), or Pathang (Korean). Any font that
is so described may be used to represent the generic 'sans-serif'
family.
 
<p>Examples of fonts that fit this description include:</p>
 
<table>
<tr><td>Latin fonts
  <td>MS Trebuchet, ITC Avant Garde Gothic, MS Arial, MS Verdana, Univers,
Futura, ITC Stone Sans, Gill Sans, Akzidenz Grotesk, Helvetica
<tr><td>Greek fonts
  <td>Attika, Typiko New Era, MS Tahoma, Monotype Gill Sans 571, Helvetica Greek
<tr><td>Cyrillic fonts
  <td>Helvetica Cyrillic, ER Univers, Lucida Sans Unicode, Bastion
<tr><td>Hebrew fonts
  <td>Arial Hebrew, MS Tahoma
<tr><td>Japanese fonts
  <td>Shin Go, Heisei Kaku Gothic W5
<tr><td>Arabic fonts
  <td>MS Tahoma
</table>
 
<h5> <span class="index-def" title="cursive, definition of">
<a name="cursive-def"><dfn>cursive</dfn></a></span></h5>
 
<p>Glyphs in cursive fonts, as the term is used in CSS, generally have
either joining strokes or other cursive characteristics beyond those
of italic typefaces. The glyphs are partially or completely
connected, and the result looks more like handwritten pen or brush
writing than printed letterwork. Fonts for some scripts, such as
Arabic, are almost always cursive. 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.
 
<p>Examples of fonts that fit this description include:</p>
 
<table>
<tr><td>Latin fonts
  <td>Caflisch Script, Adobe Poetica, Sanvito, Ex Ponto, Snell Roundhand,
Zapf-Chancery
<tr><td>Cyrillic fonts
  <td>ER Architekt
<tr><td>Hebrew fonts
  <td>Corsiva
<tr><td>Arabic fonts
  <td>DecoType Naskh, Monotype Urdu 507
</table>
 
<h5> <span class="index-def" title="fantasy, definition of">
<a name="fantasy-def"><dfn>fantasy</dfn></a></span></h5>
 
 
<p>Fantasy fonts, as used in CSS, are primarily decorative while
still containing representations of characters (as opposed to Pi or
Picture fonts, which do not represent characters). Examples include:</p>
 
<table>
<tr><td>Latin fonts
  <td>Alpha Geometrique, Critter, Cottonwood, FB Reactor, Studz
</table>
 
 
<h5> <span class="index-def" title="monospace, definition of">
<a name="monospace-def"><dfn>monospace</dfn></a></span></h5>
 
 
<p>The sole criterion of a monospace font is that all glyphs have the same fixed width. (This can make some scripts,
such as Arabic, look most peculiar.) The effect is similar to a manual
typewriter, and is often used to set samples of computer code.
 
<p>Examples of fonts which fit this description include:
 
<table>
<tr><td>Latin fonts
  <td>Courier, MS Courier New, Prestige, Everson Mono
<tr><td>Greek Fonts
  <td>MS Courier New, Everson Mono
<tr><td>Cyrillic fonts
  <td>ER Kurier, Everson Mono
<tr><td>Japanese fonts
  <td>Osaka Monospaced
<tr><td>Cherokee fonts
  <td>Everson Mono
</table>
 
<h3>15.3 <a name="font-selection">Font selection</a></h3>
 
<P>The second phase of the CSS2 font mechanism concerns the user
agent's selection of a font based on author-specified font properties,
available fonts, etc. The details of the <a href="content/view/2152/698/#algorithm">font
matching algorithm</a> are provided below.
 
<P>There are four possible font selection actions:
name matching, intelligent matching, synthesis, and download.</p>
 
<dl>
<dt><a name="x33"><span class="index-def" title="name matching"><dfn><strong>font name
matching</strong></dfn></span></a>
<dd> In this case, the user agent uses an
existing, accessible font that has the same family name as the
requested font. (Note that the appearance and the metrics might not
necessarily match, if the font that the document author used and the
font on the client system are from different foundries). The matching
information is restricted to the CSS font properties, including the
family name. This is the only method used by CSS1.
 
<dt><a name="x34"><span class="index-def" title="intelligent font
      matching"><dfn><strong>intelligent font
matching</strong>
</dfn></span></a>
<dd>In this case, the user agent uses an existing, accessible font that is
the closest match in appearance to the requested font. (Note that the
metrics might not match exactly). The matching information includes
information about the kind of font (text or symbol), nature of serifs,
weight, cap height, x height, ascent, descent, slant, etc.
 
<dt><a name="x35"><span class="index-def" title="font synthesis"><dfn>
<strong>font synthesis</strong></dfn></span></a>
<dd> In this case, the user agent creates a font that is not
only a close match in appearance, but also matches the metrics of the
requested font. The synthesizing information includes the matching
information and typically requires more accurate values for the
parameters than are used for some matching schemes. In particular,
synthesis requires accurate width metrics and character to glyph
substitution and position information if all the layout characteristics of
the specified font are to be preserved.
 
<dt><a name="x36"><span class="index-def" title="font download">
<dfn><strong>font download</strong></dfn></span></a>
<dd>Finally, the user agent may retrieve a font
over the Web. This is similar to the process of fetching images,
sounds, or applets over the Web for display in the current document,
and likewise can cause some delay before the page can be displayed.
</DL>
 
<p><a name="x37"><span class="index-def" title="progressive rendering"><dfn>Progressive
rendering</dfn></span></a> is a combination of download and one of the
other methods; it provides a temporary substitute font (using name
matching, intelligent matching, or synthesis) to allow content to be
read while the requested font downloads. Once the real font has been
successfully downloaded, it replaces the
temporary font, hopefully without the need to reflow.
 
<div class="note"><P>
<em><strong>Note.</strong>
Progressive rendering requires metric information about the font in
order to avoid re-layout of the content when the actual font has been
loaded and rendered. This metric information is sufficiently verbose
that it should only be specified at most once per font in a document.
</em>
</div>
 
 
<h4>15.3.1 <span class="index-inst" title="font description"> <a
name="font-descriptions">Font Descriptions</a></span> and
@font-face</h4>
 
<P>The font description provides the bridge between an author's font
specification and the <a name="x39"><span class="index-def" title="font
data"><dfn>font data</dfn></span></a>, which is the data needed to format
text and to render the abstract glyphs to which the characters
map - the actual scalable outlines or bitmaps. Fonts are
<em>referenced</em> by style sheet properties.
 
<P>The font description is added to the font database and then used to select the relevant font data. The
font description contains descriptors such as the location of the
font data on the Web, and characterizations of that font data. The font
descriptors are also needed to match the style sheet font properties
to particular font data. The level of detail of a font description can
vary from just the name of the font up to a list of glyph
widths. 
 
<P>Font descriptors may be classified into three types:</p>
 
<ol>
<li>those that provide the link between the CSS usage of the font and
the font description (these have the same names as the corresponding
CSS font properties),
 
<li>the URI for the location of the font data,
 
<li>those that further characterize the font, to provide a link
    between the font description and the font data.
</ol>
 
 
<p>All font descriptions are specified via a <a name="x40"><span class="index-def"
title="@font-face"><em>@font-face</em></span></a>
<a name="x41"><span class="index-inst" title="at-rules">at-rule</span></a>. The 
general form is:
 
<P><tt>@font-face { <a href="content/view/2152/698/#value-def-font-description" class="noxref"><span class="value-inst-font-description"><font-description></span></a> }</tt>
 
<p>where the <span class="index-def" title="<font-description>::definition of"><a name="value-def-font-description"><font-description></a></span> has the form:
 
<pre>
descriptor: value;
descriptor: value;
[...]
descriptor: value;
 

Each @font-face rule specifies a value for every font descriptor, either implicitly or explicitly. Those not given explicit values in the rule take the initial value listed with each descriptor in this specification. These descriptors apply solely within the context of the @font-face rule in which they are defined, and do not apply to document language elements. Thus, there is no notion of which elements the descriptors apply to, or whether the values are inherited by child elements.

The available font descriptors are described in later sections of this specification.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
  <HEAD>
    <TITLE>Font test</TITLE>
    <STYLE TYPE="text/css" MEDIA="screen, print">
      @font-face {
        font-family: "Robson Celtic";
        src: url("http://site/fonts/rob-celt")
      }
      H1 { font-family: "Robson Celtic", serif }
    </STYLE>
  </HEAD>
  <BODY>
    <H1> This heading is displayed using Robson Celtic</H1>
  </BODY>
</HTML>

CSS1 implementations, which do not understand the @font-face rule, will encounter the opening curly brackets and will ignore forward until the matching closing curly brackets. This at-rule conforms with the forward-compatible parsing requirement of CSS. Parsers may ignore these rules without error.

Having the font descriptors separate from the font data has a benefit beyond being able to do font selection and/or substitution. The data protection and replication restrictions on the font descriptors may be much weaker than on the full font data. Thus, it may be possible to install the font definition locally, or at least to have it in a local cache if it occurs in a commonly referenced style sheet; this would not require accessing the full font definition over the Web more than once per named font.

If a font descriptor is duplicated, the last occurring descriptor wins and the rest must be ignored.

Also, any descriptors that are not recognized or useful to the user agent must be ignored. Future versions of CSS may allow additional descriptors for the purpose of better font substitution, matching, or synthesis.

15.3.2 Descriptors for Selecting a Font: 'font-family', 'font-style', 'font-variant', 'font-weight', 'font-stretch' and 'font-size'

The following descriptors have the same names as the corresponding CSS2 font properties, and take a single value or comma-separated list of values.

The values within that list are, except as explicitly noted, the same as those for the corresponding CSS2 property. If there is a single value, that is the value that must be matched. If there is a list, any list item constitutes a match. If the descriptor is omitted from the @font-face, the initial value for the descriptor is used.

This is the descriptor for the font family name of a font and takes the same values as the 'font-family' property.

This is the descriptor for the style of a font and takes the same values as the 'font-style' property, except that a comma-separated list is permitted.

This is the CSS indication of whether this face is the small-caps variant of a font. It takes the same values as the 'font-variant' property except that a comma-separated list is permitted.

This is the descriptor for the weight of a face relative to others in the same font family. It takes the same values as the 'font-weight' property with three exceptions:

1. relative keywords (bolder, lighter) are not permitted.
2. a comma-separated list of values is permitted, for fonts that contain multiple weights.
3. an additional keyword, 'all' is permitted, which means that the font will match for all possible weights; either because it contains multiple weights, or because that face only has a single weight.

This is the CSS indication of the condensed or expanded nature of the face relative to others in the same font family. It takes the same values as the 'font-stretch' property except that:

• relative keywords (wider, narrower) are not permitted
• a comma-separated list is permitted
• The keyword 'all' is permitted

This is the descriptor for the sizes provided by this font. Only absolute length units are permitted, in contrast to the 'font-size' property, which allows both relative and absolute lengths and sizes. A comma-separated list of absolute lengths is permitted.

The initial value of 'all' is suitable for most scalable fonts, so this descriptor is primarily for use in an @font-face for bitmap fonts, or scalable fonts designed to be rasterised at a restricted range of font sizes.

15.3.3 Descriptors for Font Data Qualification: 'unicode-range'

The following descriptor is optional within a font definition, but is used to avoid checking or downloading a font that does not have sufficient glyphs to render a particular character.

This is the descriptor for the range of ISO 10646 characters covered by the font.

The values of <urange> are expressed using hexadecimal numbers prefixed by "U+", corresponding to character code positions in ISO 10646 ([ISO10646]).

For example, U+05D1 is the ISO 10646 character 'Hebrew letter bet'. For values outside the Basic Multilingual Plane (BMP), additional leading digits corresponding to the plane number are added, also in hexadecimal, like this: U+A1234 which is the character on Plane 10 at hexadecimal code position 1234. At the time of writing no characters had been assigned outside the BMP. Leading zeros (for example, 0000004D) are valid, but not required.

The initial value of this descriptor covers not only the entire Basic Multilingual Plane (BMP), which would be expressed as U+0-FFFF, but also the whole repertoire of ISO 10646. Thus, the initial value says that the font may have glyphs for characters anywhere in ISO 10646. Specifying a value for 'unicode-range' provides information to make searching efficient, by declaring a constrained range in which the font may have glyphs for characters. The font need not be searched for characters outside this range.

Values may be written with any number of digits. For single numbers, the character '?' is assumed to mean 'any value' which creates a range of character positions. Thus, using a single number:

unicode-range: U+20A7

no wild cards - it indicates a single character position (the Spanish peseta currency symbol)

unicode-range: U+215?

one wild card, covers the range 2150 to 215F (the fractions)

unicode-range: U+00??

two wild cards, covers the range 0000 to 00FF (Latin-1)

unicode-range: U+E??

two wild cards, covers 0E00 to 0EFF (the Lao script)

A pair of numbers in this format can be combined with the dash character to indicate larger ranges. For example:

unicode-range: U+AC00-D7FF

the range is AC00 to D7FF (the Hangul Syllables area)

Multiple, discontinuous ranges can be specified, separated by a comma. As with other comma-separated lists in CSS, any whitespace before or after the comma is ignored. For example:

unicode-range: U+370-3FF, U+1F??

This covers the range 0370 to 03FF (Modern Greek) plus 1F00 to 1FFF (Ancient polytonic Greek).

unicode-range: U+3000-303F, U+3100-312F, U+32??, U+33??, U+4E00-9FFF, U+F9000-FAFF, U+FE30-FE4F

Something of a worst case in terms of verbosity, this very precisely indicates that this (extremely large) font contains only Chinese characters from ISO 10646, without including any characters that are uniquely Japanese or Korean. The range is 3000 to 303F (CJK symbols and punctuation) plus 3100 to 312F (Bopomofo) plus 3200 to 32FF (enclosed CJK letters and months) plus 3300 to 33FF (CJK compatibility zone) plus 4E00 to 9FFF (CJK unified Ideographs) plus F900 to FAFF (CJK compatibility ideographs) plus FE30 to FE4F (CJK compatibility forms).

A more likely representation for a typical Chinese font would be:

unicode-range: U+3000-33FF, U+4E00-9FFF

unicode-range: U+11E00-121FF

This font covers a proposed registration for Aztec pictograms, covering the range 1E00 to 21FF in plane 1.

unicode-range: U+1A00-1A1F

This font covers a proposed registration for Irish Ogham covering the range 1A00 to 1A1F

15.3.4 Descriptor for Numeric Values: 'units-per-em'

The following descriptor specifies the number of "units" per em; these units may be used by several other descriptors to express various lengths, so 'units-per-em' is required if other descriptors depend on it.

This is the descriptor for the number of the coordinate units on the em square, the size of the design grid on which glyphs are laid out.

15.3.5 Descriptor for Referencing: 'src'

This descriptor is required for referencing actual font data, whether downloadable or locally installed.

This is a prioritized, comma-separated list of external references and/or locally installed font face names. The external reference points to the font data on the Web. This is required if the WebFont is to be downloaded. The font resource may be a subset of the source font, for example it may contain only the glyphs needed for the current page or for a set of pages.

The external reference consists of a URI, followed by an optional hint regarding the format of font resource to be found at that URI, and this information should be used by clients to avoid following links to fonts in formats they are unable to use. As with any hypertext reference, there may be other formats available, but the client has a better idea of what is likely to be there, in a more robust way than trying to parse filename extensions in URIs.

The format hint contains a comma-separated list of format strings that denote well-known font formats. The user agent will recognize the name of font formats that it supports, and will avoid downloading fonts in formats that it does not recognize.

An initial list of format strings defined by this specification and representing formats likely to be used by implementations on various platforms is:

As with other URIs in CSS, the URI may be partial, in which case it is resolved relative to the location of the style sheet containing the @font-face.

The locally-installed <font-face-name> is the full font name of a locally installed font. The full font name is the name of the font as reported by the operating system and is the name most likely to be used in reader style sheets, browser default style sheets or possibly author style sheets on an intranet. Adornments such as bold, italic, and underline are often used to differentiate faces within a font family. For more information about full font names please consult the notes below.

The notation for a <font-face-name> is the full font name, which must be quoted since it may contain any character, including spaces and punctuation, and also must be enclosed in "local(" and ")".

Access to locally installed fonts is via the <font-face-name>. The font face name is not truly unique, nor is it truly platform or font format independent, but at the moment it is the best way to identify locally installed font data. The use of the font face name can be made more accurate by providing an indication of the glyph complement required. This may be done by indicating the range of ISO 10646 character positions for which the font provides some glyphs (see 'unicode-range').

15.3.6 Descriptors for Matching: 'panose-1', 'stemv', 'stemh', 'slope', 'cap-height', 'x-height', 'ascent', and 'descent'

These descriptors are optional for a CSS2 definition, but may be used if intelligent font matching or font size adjustment is desired by the author.

This is the descriptor for the Panose-1 number and consists of ten decimal integers, separated by whitespace. A comma-separated list is not permitted for this descriptor, because the Panose-1 system can indicate that a range of values are matched. The initial value is zero, which means "any", for each PANOSE digit; all fonts will match the Panose number if this value is used. Use of the Panose-1 descriptor is strongly recommended for latin fonts. For further details, see Appendix C.

This is the descriptor for the vertical stem width of the font. If the value is undefined, the descriptor is not used for matching. If this descriptor is used, the 'units-per-em' descriptor must also be used.

This is the descriptor for the horizontal stem width of the font. If the value is undefined, the descriptor is not used for matching. If this descriptor is used, the 'units-per-em' descriptor must also be used.

This is the descriptor for the vertical stroke angle of the font.

This is the descriptor for the number of the height of uppercase glyphs of the font. If the value is undefined, the descriptor is not used for matching. If this descriptor is used, the 'units-per-em' descriptor must also be used.

This is the descriptor for the height of lowercase glyphs of the font. If the value is undefined, the descriptor is not used for matching. If this descriptor is used, the 'units-per-em' descriptor must also be used. This descriptor can be very useful when using the 'font-size-adjust' property, because computation of the z value of candidate fonts requires both the font size and the x-height; it is therefore recommended to include this descriptor.

This is the descriptor for the maximum unaccented height of the font. If the value is undefined, the descriptor is not used for matching. If this descriptor is used, the 'units-per-em' descriptor must also be used.

This is the descriptor for the Maximum unaccented depth of the font. If the value is undefined, the descriptor is not used for matching. If this descriptor is used, the 'units-per-em' descriptor must also be used.

15.3.7 Descriptors for Synthesis: 'widths', 'bbox' and 'definition-src'

Synthesizing a font means, at minimum, matching the width metrics of the specified font. Therefore, for synthesis, this metric information must be available. Similarly, progressive rendering requires width metrics in order to avoid reflow of the content when the actual font has been loaded. Although the following descriptors are optional for a CSS2 definition, some are required if synthesizing (or reflow-free progressive rendering) is desired by the author. Should the actual font become available, the substitute should be replaced by the actual font. Any of these descriptors that are present will be used t