SVG Tiny 1.2 – 20081222

- Top
- Contents
- Previous
- Next
- Elements
- Attributes

# 17 Fonts

## Contents

## 17.1 Introduction

Reliable delivery of fonts is a requirement for SVG. Designers need to create SVG content with arbitrary fonts and know that the same graphical result will appear when the content is viewed by all end users, even when end users do not have the necessary fonts installed on their computers. This parallels the print world, where the designer uses a given font when authoring a drawing for print, and the graphical content appears exactly the same in the printed version as it appeared on the designer's authoring system.

Historically, one approach has been to convert all text to paths representing the glyphs used. This preserves the visual look, but means that the text is lost; it cannot be dynamically updated and is not accessible. Another approach is to hope that a font with a given name is available to all renderers. This assumption does not work well on the desktop and works very badly in a heterogeneous mobile environment. SVG solves this problem by allowing the text to be converted to paths, but storing those paths as an SVG font. The text is retained and remains dynamically modifiable and accessible.

### 17.1.1 Describing fonts available to SVG

SVG utilizes an XML version of the WebFonts facility defined in the "Cascading Style Sheets (CSS) level 2" specification [CSS2] to reference fonts. Described fonts may be in a variety of different formats. By describing key details of the font such as its family name, weight, whether it is italic and so on, text can continue to use the font properties without having to explicitly indicate the font that is to be used for each span of text.

### 17.1.2 Defining fonts in SVG

One disadvantage to the WebFont facility to date is that specifications such as [CSS2] do not require support of particular font formats. The result is that different implementations support different Web font formats, thereby making it difficult for Web site creators to post a single Web site using WebFonts that work across all user agents.

To provide a common font format for SVG that is guaranteed to be supported by all conforming SVG viewers, SVG also defines a font format, in SVG, which uses the same geometric mechanism for glyphs as is used by the SVG path element. This facility is called SVG fonts. SVG implementations must support the SVG font format, and may also support other formats. WebFonts may be contained in the SVG document which uses them, or stored in a separate document (for example, to allow sharing of the same font by multiple SVG documents).

Taken together, these two mechanisms ensure reliable delivery of font data to end users, preserving graphical richness and enabling accessible access to the textual data. In a common scenario, SVG authoring applications generate compressed, subsetted WebFonts for all text elements used by a given SVG document fragment. SVG fonts can improve the semantic richness of graphics that represent text. For example, many company logos consist of the company name drawn artistically. In some cases, accessibility may be enhanced by expressing the logo as a series of glyphs in an SVG font and then rendering the logo as a 'text' element which references this font.

## 17.2 Overview of SVG fonts

An SVG font is a font defined using SVG's 'font' element.

The purpose of SVG fonts is to allow for delivery of glyph outlines in display-only environments. SVG fonts that accompany Web pages must be supported only in browsing and viewing situations. Graphics editing applications or file translation tools must not attempt to convert SVG fonts into system fonts.

SVG fonts contain unhinted font outlines. Because of this, on many implementations there will be limitations regarding the quality and legibility of text in small font sizes. For increased quality and legibility in small font sizes, content creators may want to use an alternate font technology, such as fonts that ship with operating systems or an alternate WebFont format.

Because SVG fonts are expressed using SVG elements and attributes, in some cases the SVG font will take up more space than if the font were expressed in a different format which was especially designed for compact expression of font data. For the fastest delivery of Web pages, content creators may want to use an alternate font technology as a first choice, with a fallback to an SVG font for interoperability.

A key value of SVG fonts is guaranteed availability in SVG user agents. In some situations, it might be appropriate for an SVG font to be the first choice for rendering some text. In other situations, the SVG font might be an alternate, back-up font in case the first choice font (perhaps a hinted system font) is not available to a given user.

The characteristics and attributes of SVG fonts correspond closely to the font characteristics and parameters described in the "Fonts" chapter of the "Cascading Style Sheets (CSS) level 2" specification [CSS2]. In this model, various font metrics, such as advance values and baseline locations, and the glyph outlines themselves, are expressed in units that are relative to an abstract square whose height is the intended distance between lines of type in the same type size. This square is called the em square and it is the design grid on which the glyph outlines are defined. The value of the 'units-per-em' attribute on the 'font' element specifies how many units the em square is divided into. Common values for other font types are, for example, 250 (Intellifont), 1000 (Type 1) and 2048 (TrueType, TrueType GX and Open-Type). Unlike standard graphics in SVG, where the initial coordinate system has the y-axis pointing downward (see The initial coordinate system), the design grid for SVG fonts, along with the initial coordinate system for the glyphs, has the y-axis pointing upward for consistency with accepted industry practice for many popular font formats.

SVG fonts and their associated glyphs do not specify bounding box information. Because the glyph outlines are expressed as SVG path elements, the implementation has the option to render the glyphs either using standard graphics calls or by using special-purpose font rendering technology, in which case any necessary maximum bounding box and overhang calculations can be performed from analysis of the path elements contained within the glyph outlines.

An SVG font can be either embedded within the same document that uses the font or saved as part of an external resource.

Here is an example of how you might embed an SVG font inside of an SVG document.

Example: 20_01.svg

    <?xml version="1.0"?>
    <svg width="400px" height="300px" version="1.2" baseProfile="tiny"
      xmlns = 'http://www.w3.org/2000/svg'>
      <defs>
        <font xml:id="Font1" horiz-adv-x="1000">
          <font-face font-family="Super Sans" font-weight="bold" font-style="normal"
              units-per-em="1000" cap-height="600" x-height="400"
              ascent="700" descent="300" alphabetic="0" mathematical="350" ideographic="400" hanging="500">
          </font-face>
          <missing-glyph d="M0,0h200v200h-200z"/>
          <glyph unicode="!" horiz-adv-x="300" d="--Outline of exclam. pt. glyph--"/>
          <glyph unicode="@" d="--Outline of @ glyph--"/>
          <!-- more glyphs -->
        </font>
      </defs>
      <desc>An example using an embedded font.</desc>
      <text x="100" y="100" font-family="Super Sans, Helvetica, sans-serif"
                      font-weight="bold" font-style="normal">Text
        using embedded font</text>
    </svg>

## 17.3 The 'font' element

The 'font' element defines an SVG font.

Schema: font

        <define name='font'>
          <element name='font'>
            <ref name='font.AT'/>
            <ref name='font.CM'/>
          </element>
        </define>

        <define name='font.AT' combine='interleave'>
          <ref name='svg.Core.attr'/>
          <ref name='svg.External.attr'/>
          <ref name='svg.FontAdvOrigCommon.attr'/>
          <optional>
            <attribute name='horiz-origin-x' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
        </define>

        <define name='font.CM'>
          <zeroOrMore>
            <choice>
              <ref name='svg.Desc.group'/>
              <ref name='font-face'/>
              <ref name='missing-glyph'/>
              <ref name='glyph'/>
              <ref name='hkern'/>
            </choice>
          </zeroOrMore>
        </define>

Attribute definitions:

horiz-origin-x = "<number>"

  The X-coordinate in the font coordinate system of the origin of a glyph to be used when drawing horizontally oriented text. (Note that the origin applies to all glyphs in the font.)

  The lacuna value is '0'.

  Animatable: no.

horiz-adv-x = "<number>"

  The default horizontal advance after rendering a glyph in horizontal orientation. Glyph widths are required to be non-negative, even if the glyph is typically rendered right-to-left, as in Hebrew and Arabic scripts.

  The lacuna value is '0'.

  Animatable: no.

Each 'font' element must have a 'font-face' child element which describes various characteristics of the font.

## 17.4 The 'glyph' element

The 'glyph' element defines the graphics for a given glyph. The coordinate system for the glyph is defined by the various attributes in the 'font' element.

The graphics that make up the 'glyph' consist of a single path data specification within the 'd' attribute.

Schema: glyph

        <define name='glyph'>
          <element name='glyph'>
            <ref name='glyph.AT'/>
            <ref name='glyph.CM'/>
          </element>
        </define>

        <define name='glyph.AT' combine='interleave'>
          <ref name='svg.Core.attr'/>
          <ref name='svg.FontAdvOrigCommon.attr'/>
          <ref name='svg.D.attr'/>
          <optional>
            <attribute name='unicode' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional><attribute name='glyph-name' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional><attribute name='arabic-form' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional>
            <attribute name='lang' svg:animatable='false' svg:inheritable='false'>
              <ref name='LanguageIDs.datatype'/>
            </attribute>
          </optional>
        </define>

        <define name='glyph.CM'>
          <zeroOrMore>
            <choice>
              <ref name='svg.Desc.group'/>
            </choice>
          </zeroOrMore>
        </define>

Attribute definitions:

unicode = "<string>"

  One or more characters indicating the sequence of characters which corresponds to this glyph. If a single character is provided, then this glyph corresponds to the given Unicode character. If multiple characters are provided, then this glyph corresponds to the given sequence of Unicode characters. One use of a sequence of characters is ligatures. For example, if unicode="ffl", then the given glyph will be used to render the sequence of characters "f", "f", and "l".

  It is often useful to refer to characters using XML character references expressed in hexadecimal notation or decimal notation. For example, unicode="ffl" could be expressed as XML character references in hexadecimal notation as unicode="&#x66;&#x66;&#x6c;" or in decimal notation as unicode="&#102;&#102;&#108;".

  The 'unicode' attribute contributes to the process for deciding which glyph(s) are used to represent which character(s). See glyph selection rules. If the 'unicode' attribute is not provided for a given 'glyph', the glyph cannot be accessed in SVG Tiny 1.2.

  Animatable: no.

glyph-name = "<list-of-strings>"

  An optional name for the glyph. Glyph names should be unique within a font. The glyph names can be used in situations where Unicode character numbers do not provide sufficient information to access the correct glyph, such as when there are multiple glyphs per Unicode character. The glyph names can be referenced in kerning definitions.

  Animatable: no.

d = "path data"

  The definition of the outline of a glyph, using the same syntax as for the 'd' attribute on a 'path' element. See Path data.

  See below for a discussion of this attribute.

  Animatable: no.

arabic-form = "initial | medial | terminal | isolated"

  For Arabic glyphs, indicates which of the four possible forms this glyph represents.If arabic-form is not specified for a glyph that requires it, the glyph is taken to be the isolated form and the initial, medial, and terminal forms will render with missing-glyph unless separately specified.

  Additionally, if initial, medial, or terminal are specified on a glyph that does not require the arabic-form to be specified, the arabic-form attribute shall have no effect.

  Animatable: no.

lang = "<list-of-language-ids>"

  The attribute value is a comma-separated list of language tags as defined in IETF Best Current Practice 47 [BCP47]. For XML content, the glyph can be used if the 'xml:lang' attribute exactly matches one of the languages given in the value of this parameter, or if the 'xml:lang' attribute exactly equals a prefix of one of the languages given in the value of this parameter such that the first tag character following the prefix is "-". If the attribute is not specified, then the glyph can be used in all languages.

  For example, a glyph suitable for French (whose lang attribute includes the language tag 'fr') is suitable for all types of French text (for example, Canadian French, xml:lang="fr-ca"). A glyph whose lang attribute is specific for Traditional Chinese (lang includes zh-Hant) is not suitable for Simplified Chinese (xml:lang="zh-Hanc") or generic Chinese (xml:lang="zh").

  Animatable: no.

horiz-adv-x = "<number>"

  The horizontal advance after rendering the glyph in horizontal orientation. If the attribute is not specified, the effect is as if the attribute were set to the value of the font's 'horiz-adv-x' attribute.

  Glyph widths are required to be non-negative, even if the glyph is typically rendered right-to-left, as in Hebrew and Arabic scripts.

  Animatable: no.

The graphics for the 'glyph' are specified using the 'd' attribute. The path data within this attribute must be processed as follows:

- Any relative coordinates within the path data specification are converted into equivalent absolute coordinates.

- Each of these absolute coordinates is transformed from the font coordinate system into the text content element's current coordinate system such that the origin of the font coordinate system is properly positioned and rotated to align with the current text position and orientation for the glyph, and scaled so that the correct 'font-size' is achieved.

- The resulting, transformed path specification is rendered as if it were a 'path' element, using the styling properties that apply to the characters which correspond to the given glyph, and ignoring any styling properties specified on the 'font' element or the 'glyph' element.

In general, the 'd' attribute renders in the same manner as system fonts. For example, a dashed pattern will usually look the same if applied to a system font or to an SVG font which defines its glyphs using the 'd' attribute. Many implementations will be able to render glyphs quickly and will be able to use a font cache for further performance gains.

Example font01 below contains a font for just three letters - S, V, and G - plus a missing glyph for all other characters. There is also a kern pair, to bring the V and the G glyphs closer together. The font, Anglepoise, was designed by Ray Larabie of Larabie Fonts and is used by permission.

Example: font01.svg

    <?xml version="1.0" encoding="UTF-8"?>
    <svg version="1.2" baseProfile="tiny" viewBox="0 0 160 70"  xmlns="http://www.w3.org/2000/svg"
        xmlns:xlink="http://www.w3.org/1999/xlink">
        <title>Font example</title>
        <defs>
            <font horiz-adv-x="313" xml:id="la">
                <metadata>Converted from Larabie Anglepoise by Batik ttf2svg
                See http://www.larabiefonts.com/ </metadata>
                <font-face font-family="larabie-anglepoise" units-per-em="1000"
                    panose-1="0 0 4 0 0 0 0 0 0 0" ascent="703" descent="-300" alphabetic="0"/>
                <missing-glyph horiz-adv-x="500" d="M63 0V700H438V0H63ZM125 63H375V638H125V63Z"/>
                <glyph unicode="S" glyph-name="S" horiz-adv-x="385" d="M371 1H29V144H264Q264 151 264
                    166Q265 180 265 188Q265 212 249 212H132Q83 212 55 247Q29 279 29
                    329V566H335V422H136V375Q136 360 144 356Q148 355 168 355H279Q327 355 352 309Q371 273
                    371 221V1Z"/>
                <glyph unicode="V" glyph-name="V" horiz-adv-x="351" d="M365 563L183 -33L0 563H101L183
                    296L270 563H365Z"/>
                <glyph unicode="G" glyph-name="G" horiz-adv-x="367" d="M355
                    1H18V564H355V420H125V144H248V211H156V355H355V1Z"/>
                <hkern g1="V" g2="G" k="-40"/>
            </font>
        </defs>
        <text x="40" y="50" font-family="larabie-anglepoise" font-size="70" fill="#933">SVG</text>
        <rect x="00" y="00" width="160" height="70" stroke="#777" fill="none"/>
    </svg>

Example font02 below contains a font for a single character, the Arabic letter خ - plus a space and missing glyph. There are four glyphs for this character, each corresponding to the same Unicode code point U+062E. They are distinguished by the values of the arabic-form attribute. The text string demonstrates each of the four forms.

Example: font02.svg

    <?xml version="1.0" encoding="UTF-8"?>
    <svg version="1.2" baseProfile="tiny" viewBox="80 100 260 180" xmlns="http://www.w3.org/2000/svg"
        xmlns:xlink="http://www.w3.org/1999/xlink">
        <title>Font example for arabic-form</title>
        <defs>
            <font horiz-adv-x="573">
                <font-face font-family="SVGAr" units-per-em="1000" panose-1="5 1 1 1 1 1 1 1 1 1"
                    ascent="1025" descent="-399" alphabetic="0"/>
                <missing-glyph horiz-adv-x="500" d="M31 0V800H469V0H31ZM438 31V769H62V31H438Z"/>
                <glyph unicode=" " glyph-name="space" horiz-adv-x="370"/>
                <glyph unicode="&#x62e;" glyph-name="khah-isolated" arabic-form="isolated"
                    horiz-adv-x="562" d="M309 360Q309 353 297 335T271 317Q260 317 227 337T194 370Q194
                    380 205 397T232 415Q246 415 277 395T309 360ZM518 -265Q516 -269 509 -275Q507 -277 502
                    -281Q447 -319 424 -330Q356 -363 281 -363Q228 -363 186 -347T110 -294Q69 -249 54
                    -199Q44 -167 44 -127Q44 -96 50 -65T76 12Q88 39 110 70Q152 127 152 137Q152 151 148
                    156T121 161Q95 161 85 156Q72 146 62 140Q44 128 35 130Q35 138 35 146Q37 151 43 162Q77
                    208 98 219T159 231Q170 231 234 221Q255 218 298 210H340Q347 210 382 218T425 230T435
                    235Q446 239 449 234Q454 226 444 189T426 152Q418 152 393 154T360 156Q327 156 297
                    149T228 120Q180 93 142 36Q96 -33 96 -110Q96 -209 168 -257Q223 -294 300 -294Q343 -294
                    371 -291Q429 -285 489 -267Q506 -260 511 -260Q514 -262 518 -265Z"/>
                <glyph unicode="&#x62e;" glyph-name="khah-initial" arabic-form="initial"
                    horiz-adv-x="728" d="M297 372Q297 365 285 347T259 329Q248 329 215 349T182 382Q182
                    392 193 409T220 427Q234 427 265 407T297 372ZM600 0H0V68H373Q396 68 396 86Q396 89 394
                    95Q377 137 347 159Q308 188 243 188Q210 188 183 171Q165 160 157 158T145 156Q138 156
                    138 164L140 174Q145 196 191 220Q228 240 269 240Q313 240 355 221T447 160Q488 120 530
                    81Q543 73 563 71T609 68Q619 68 620 68T625 68Q645 68 645 46Q645 30 633 15T600 0Z"/>
                <glyph unicode="&#x62e;" glyph-name="khah-medial" arabic-form="medial"
                    horiz-adv-x="625" d="M296 376Q296 369 284 351T258 333Q247 333 214 353T181 386Q181
                    396 192 413T219 431Q233 431 264 411T296 376ZM625 0H0V68H373Q396 68 396 86Q396 89 394
                    95Q377 137 347 159Q308 188 243 188Q210 188 183 171Q165 160 157 158T145 156Q138 156
                    138 164L140 174Q145 196 191 220Q228 240 269 240Q313 240 355 221T447 160Q488 120 530
                    81Q543 73 563 71T609 68Q619 68 620 68T625 68V0Z"/>
                <glyph unicode="&#x62e;" glyph-name="khah-terminal" arabic-form="terminal"
                    horiz-adv-x="514" d="M296 352Q296 345 284 327T258 309Q247 309 214 329T181 362Q181
                    372 192 389T219 407Q233 407 264 387T296 352ZM514 0H375Q313 0 298 64T261 128Q209 128
                    153 62Q91 -12 91 -101Q91 -199 162 -243Q220 -279 319 -279Q367 -279 390 -276T463
                    -259Q466 -258 475 -255T488 -252Q490 -252 491 -254T489 -263Q484 -272 466 -286T433
                    -307Q408 -320 401 -323Q349 -344 277 -344Q169 -344 104 -274Q44 -210 44 -118Q44 -88 51
                    -53T73 14Q80 31 97 56Q132 108 132 118Q132 127 126 134T110 141Q92 141 85 137Q72 127
                    59 117Q49 112 44 112Q38 112 38 119Q38 122 40 128Q49 156 80 182Q116 212 157 212Q170
                    212 188 208Q232 198 258 198H320Q345 198 357 201Q374 207 383 209T398 214T412 216Q420
                    216 421 212Q424 202 414 170T396 137Q394 137 382 140T362 143Q346 143 337 135T327
                    104Q327 91 341 80T379 68H514V0Z"/>
            </font>
        </defs>
        <g font-family="SVGAr" font-size="80">
            <!-- this should produce isolated khah, followed by initial,medial and terminal khah -->
            <text x="100" y="200">&#x62e; &#x62e;&#x62e;&#x62e;</text>
            <rect x="80" y="100" width="260" height="180" fill="none" stroke="#777"/>
         </g>
    </svg>

## 17.5 The 'missing-glyph' element

The 'missing-glyph' element defines a graphic to use if there is an attempt to draw a glyph from a given font and the given glyph has not been defined. The attributes on the 'missing-glyph' element have the same meaning as the corresponding attributes on the 'glyph' element.

Schema: missing-glyph

        <define name='missing-glyph'>
          <element name='missing-glyph'>
            <ref name='missing-glyph.AT'/>
            <ref name='missing-glyph.CM'/>
          </element>
        </define>

        <define name='missing-glyph.AT' combine='interleave'>
          <ref name='svg.Core.attr'/>
          <ref name='svg.FontAdvOrigCommon.attr'/>
          <ref name='svg.D.attr'/>
        </define>

        <define name='missing-glyph.CM'>
          <zeroOrMore>
            <choice>
              <ref name='svg.Desc.group'/>
            </choice>
          </zeroOrMore>
        </define>

## 17.6 Glyph selection rules

When determining the glyph(s) to draw a given character sequence, the 'font' element must be searched from its first 'glyph' element to its last in logical order to see if the upcoming sequence of Unicode characters to be rendered matches the sequence of Unicode characters specified in the 'unicode' attribute for the given 'glyph' element. The first successful match must be used. Thus, if an "ffl" ligature is defined in the font after the "f" glyph, it will not be used.

## 17.7 The 'hkern' element

The 'hkern' element defines kerning pairs for horizontally-oriented pairs of glyphs.

Kern pairs identify pairs of glyphs within a single font whose inter-glyph spacing is adjusted when the pair of glyphs are rendered next to each other. In addition to the requirement that the pair of glyphs are from the same font, SVG font kerning happens only when the two glyphs correspond to characters which have the same values for properties 'font-family', 'font-size', 'font-style' and 'font-weight'.

An example of a kerning pair are the letters "Va", where the typographic result might look better if the letters "V" and the "a" were rendered slightly closer together.

Right-to-left and bidirectional text in SVG is laid out in a two-step process, which is described in Relationship with bidirectionality. If SVG fonts are used, before kerning is applied, characters are re-ordered into left-to-right (or top-to-bottom, for vertical text) visual rendering order. Kerning from SVG fonts is then applied on pairs of glyphs which are rendered contiguously. The first glyph in the kerning pair is the left (or top) glyph in visual rendering order. The second glyph in the kerning pair is the right (or bottom) glyph in the pair.

For convenience to font designers and to minimize file sizes, a single 'hkern' can define a single kerning adjustment value between one set of glyphs (sequences and/or ranges of Unicode characters) and another set of glyphs (e.g., another range of Unicode characters).

The 'hkern' element defines kerning pairs and adjustment values in the horizontal advance value when drawing pairs of glyphs which the two glyphs are contiguous and are both rendered horizontally (i.e., side-by-side). The spacing between characters is reduced by the kerning adjustment. (Negative kerning adjustments increase the spacing between characters.)

Schema: hkern

        <define name='hkern'>
          <element name='hkern'>
            <ref name='hkern.AT'/>
            <ref name='hkern.CM'/>
          </element>
        </define>

        <define name='hkern.AT' combine='interleave'>
          <ref name='svg.Core.attr'/>
          <optional>
            <attribute name='u1' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional>
            <attribute name='g1' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional>
            <attribute name='u2' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional>
            <attribute name='g2' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional>
            <attribute name='k' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
        </define>

        <define name='hkern.CM'>
          <zeroOrMore>
            <choice>
              <ref name='svg.Desc.group'/>
            </choice>
          </zeroOrMore>
        </define>

Attribute definitions:

u1 = "[<Char> | <urange> ] [, [<Char> | <urange>] ]* "

  A sequence (comma-separated) of characters and/or ranges of Unicode characters (see description of ranges of Unicode characters in [CSS2]) which identify a set of possible first glyphs in the kerning pair. If a given Unicode character within the set has multiple corresponding 'glyph' elements (i.e., there are multiple 'glyph' elements with the same 'unicode' attribute value, but different 'glyph-name' values), then all such glyphs are included in the set. Comma is the separator character; thus, to kern a comma, specify the comma as part of a range of Unicode characters or as a glyph name using the 'g1' attribute. The total set of possible first glyphs in the kerning pair is the union of glyphs specified by the 'u1' and 'g1' attributes.

  Animatable: no.

g1 = "<list-of-strings>"

  A sequence of glyph names (i.e., values that match 'glyph-name' attributes on 'glyph' elements) which identify a set of possible first glyphs in the kerning pair. All glyphs with the given glyph name are included in the set. The total set of possible first glyphs in the kerning pair is the union of glyphs specified by the 'u1' and 'g1' attributes.

  Animatable: no.

u2 = "[<Char> | <urange> ] [, [<Char> | <urange>] ]* "

  Same as the 'u1' attribute, except that 'u2' specifies possible second glyphs in the kerning pair.

  Animatable: no.

g2 = "<list-of-strings>"

  Same as the 'g1' attribute, except that 'g2' specifies possible second glyphs in the kerning pair.

  Animatable: no.

k = "<number>"

  The amount to decrease the spacing between the two glyphs in the kerning pair. The value is in the font coordinate system. The lacuna value is '0'.

  Animatable: no.

At least one of 'u1' and 'g1' must be provided, and at least one of 'u2' and 'g2' must be provided.

## 17.8 Describing a font

### 17.8.1 Overview of font descriptions

A font description provides the bridge between an author's font specification and the font data, 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 referenced by properties, such as the 'font-family' property.

Each specified font description is added to the font database so that it can be 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 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.

For more about font descriptions, refer to the Fonts chapter in the CSS2 specification [CSS2].

### 17.8.2 The 'font-face' element

The 'font-face' element is an XML structure which corresponds directly to the @font-face facility in CSS2. It can be used to describe the characteristics of any font, SVG font or otherwise.

When used to describe the characteristics of an SVG font contained within the same document, it is recommended that the 'font-face' element be a child of the 'font' element it is describing so that the 'font' element can be self-contained and fully-described. In this case, any 'font-face-src' elements within the 'font-face' element are ignored as it is assumed that the 'font-face' element is describing the characteristics of its parent 'font' element.

Schema: font-face

        <define name='font-face'>
          <element name='font-face'>
            <ref name='font-face.AT'/>
            <ref name='font-face.CM'/>
          </element>
        </define>

        <define name='font-face.AT' combine='interleave'>
          <ref name='svg.Core.attr'/>
          <ref name='svg.External.attr'/>
          <optional><attribute name='font-family' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional><attribute name='font-style' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional><attribute name='font-weight' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional><attribute name='font-variant' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional><attribute name='font-stretch' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional><attribute name='unicode-range' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional><attribute name='panose-1' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional><attribute name='widths' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional><attribute name='bbox' svg:animatable='false' svg:inheritable='false'><text/></attribute>
          </optional>
          <optional>
            <attribute name='units-per-em' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='stemv' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='stemh' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='slope' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='cap-height' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='x-height' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='accent-height' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='ascent' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='descent' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='ideographic' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='alphabetic' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='mathematical' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='hanging' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='underline-position' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='underline-thickness' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='strikethrough-position' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='strikethrough-thickness' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='overline-position' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
          <optional>
            <attribute name='overline-thickness' svg:animatable='false' svg:inheritable='false'>
              <ref name='Number.datatype'/>
            </attribute>
          </optional>
        </define>

        <define name='font-face.CM'>
          <zeroOrMore>
            <choice>
              <ref name='svg.Desc.group'/>
              <ref name='font-face-src'/>
            </choice>
          </zeroOrMore>
        </define>

Attribute definitions:

font-family = "<string>"

  Same syntax and semantics as the 'font-family' descriptor within an @font-face rule.

  Animatable: no.

font-style = "all | [ normal | italic | oblique] [, [normal | italic | oblique]]*"

  Same syntax and semantics as the 'font-style' descriptor within an @font-face rule. The style of a font. Takes on the same values as the 'font-style' property, except that a comma-separated list is permitted.

  The lacuna value is 'all'.

  Animatable: no.

font-weight = "all | [normal | bold |100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900] [, [normal | bold |100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900]]*"

  Same syntax and semantics as the 'font-weight' descriptor within an @font-face rule.

  The weight of a face relative to others in the same font family. Takes on the same values as the 'font-weight' property with three exceptions:

  - relative keywords (bolder, lighter) must not be used
  - a comma-separated list of values may be used, for fonts that contain multiple weights
  - an additional keyword, 'all', may be used. 'all' indicates that the font will match for all possible weights; either because it contains multiple weights, or because that face only has a single weight.

  The lacuna value is 'all'.

  Animatable: no.

unicode-range = "<urange> [, <urange>]*"

  Same syntax and semantics as the 'unicode-range' descriptor within an @font-face rule. The range of ISO 10646 characters [UNICODE] possibly covered by the glyphs in the font. Except for any additional information provided in this specification, the normative definition of the attribute is in [CSS2].

  The lacuna value is 'U+0-10FFFF'.

  Animatable: no.

units-per-em = "<number>"

  Same syntax and semantics as the 'units-per-em' descriptor within an @font-face rule. The number of coordinate units on the em square, the size of the design grid on which glyphs are laid out.

  This value should be specified as nearly every other attribute requires the definition of a design grid.

  The lacuna value is '1000'.

  Animatable: no.

panose-1 = "[<integer>]{10}"

  Same syntax and semantics as the 'panose-1' descriptor within an @font-face rule. The Panose-1 number, consisting of ten decimal integers, separated by white space. Except for any additional information provided in this specification, the normative definition of the attribute is in [CSS2].

  The lacuna value is '0 0 0 0 0 0 0 0 0 0'.

  Animatable: no.

stemv = "<number>"

  Same syntax and semantics as the 'stemv' descriptor within an @font-face rule.

  Animatable: no.

stemh = "<number>"

  Same syntax and semantics as the 'stemh' descriptor within an @font-face rule.

  Animatable: no.

slope = "<number>"

  Same syntax and semantics as the 'slope' descriptor within an @font-face rule. The vertical stroke angle of the font. Except for any additional information provided in this specification, the normative definition of the attribute is in [CSS2].

  The lacuna value is '0'.

  Animatable: no.

cap-height = "<number>"

  Same syntax and semantics as the 'cap-height' descriptor within an @font-face rule. The height of uppercase glyphs in the font within the font coordinate system.

  Animatable: no.

x-height = "<number>"

  Same syntax and semantics as the 'x-height' descriptor within an @font-face rule. The height of lowercase glyphs in the font within the font coordinate system.

  Animatable: no.

accent-height = "<number>"

  The distance from the origin to the top of accent characters, measured by a distance within the font coordinate system.

  If the attribute is not specified, the effect is as if the attribute were set to the value of the 'ascent' attribute. If this attribute is used, the 'units-per-em' attribute must also be specified.

  Animatable: no.

ascent = "<number>"

  Same syntax and semantics as the 'ascent' descriptor within an @font-face rule. The maximum unaccented height of the font within the font coordinate system.

  If the attribute is not specified, the effect is as if the attribute were set to the difference between the 'units-per-em' value and the 'vert-origin-y' value for the corresponding font.

  Animatable: no.

descent = "<number>"

  Same syntax and semantics as the 'descent' descriptor within an @font-face rule. The maximum unaccented depth of the font within the font coordinate system.

  If the attribute is not specified, the effect is as if the attribute were set to the 'vert-origin-y' value for the corresponding font.

  Animatable: no.

widths = "<string>"

  Same syntax and semantics as the 'widths' descriptor within an @font-face rule.

  Animatable: no.

bbox = "<string>"

  Same syntax and semantics as the 'bbox' descriptor within an @font-face rule.

  Animatable: no.

ideographic = "<number>"

  For horizontally oriented glyph layouts, indicates the alignment coordinate for glyphs to achieve ideographic baseline alignment. The value is an offset in the font coordinate system. If this attribute is provided, the 'units-per-em' attribute must also be specified.

  Animatable: no.

alphabetic = "<number>"

  Same syntax and semantics as the 'baseline' descriptor within an @font-face rule. For horizontally oriented glyph layouts, indicates the alignment coordinate for glyphs to achieve alphabetic baseline alignment. The value is an offset in the font coordinate system. If this attribute is provided, the 'units-per-em' attribute must also be specified.

  Animatable: no.

mathematical = "<number>"

  Same syntax and semantics as the 'mathline' descriptor within an @font-face rule. For horizontally oriented glyph layouts, indicates the alignment coordinate for glyphs to achieve mathematical baseline alignment. The value is an offset in the font coordinate system. If this attribute is provided, the 'units-per-em' attribute must also be specified.

  Animatable: no.

hanging = "<number>"

  For horizontally oriented glyph layouts, indicates the alignment coordinate for glyphs to achieve hanging baseline alignment. The value is an offset in the font coordinate system. If this attribute is provided, the 'units-per-em' attribute must also be specified.

  Animatable: no.

underline-position = "<number>"

  The ideal position of an underline within the font coordinate system. If this attribute is provided, the 'units-per-em' attribute must also be specified.

  Animatable: no.

underline-thickness = "<number>"

  The ideal thickness of an underline, expressed as a length within the font coordinate system. If this attribute is provided, the 'units-per-em' attribute must also be specified.

  Animatable: no.

strikethrough-position = "<number>"

  The ideal position of a strike-through within the font coordinate system. If this attribute is provided, the 'units-per-em' attribute must also be specified.

  Animatable: no.

strikethrough-thickness = "<number>"

  The ideal thickness of a strike-through, expressed as a length within the font coordinate system. If this attribute is provided, the 'units-per-em' attribute must also be specified.

  Animatable: no.

overline-position = "<number>"

  The ideal position of an overline within the font coordinate system. If this attribute is provided, the 'units-per-em' attribute must also be specified.

  Animatable: no.

overline-thickness = "<number>"

  The ideal thickness of an overline, expressed as a length within the font coordinate system. If this attribute is provided, the 'units-per-em' attribute must also be specified.

  Animatable: no.

### 17.8.3 The 'font-face-src' element

The 'font-face-src' element, together with the 'font-face-uri' elements described further down correspond to the 'src' descriptor within an @font-face rule. (Refer to the descriptions of the @font-face rule and 'src' descriptor in the CSS 2 specification ([CSS2], sections 15.3.1 and 15.3.5).

A 'font-face-src' element contains a 'font-face-uri' element. The 'font-face-src' element is used for referencing fonts defined elsewhere.

Schema: font-face-src

        <define name='font-face-src'>
          <element name='font-face-src'>
            <ref name='font-face-src.AT'/>
            <ref name='font-face-src.CM'/>
          </element>
        </define>

        <define name='font-face-src.AT' combine='interleave'>
          <ref name='svg.Core.attr'/>
        </define>

        <define name='font-face-src.CM'>
          <zeroOrMore>
            <choice>
              <ref name='svg.Desc.group'/>
              <ref name='font-face-uri'/>
            </choice>
          </zeroOrMore>
        </define>

### 17.8.4 The 'font-face-uri' element

The 'font-face-uri' element is used within a 'font-face-src' element to reference a font defined inside or outside of the current SVG document.

When a 'font-face-uri' is referencing an SVG font, then that reference must be to an SVG 'font' element, therefore requiring the use of a fragment identifier (see [IRI]). The referenced 'font' element can be local (i.e., within the same document as the 'font-face-uri' element) or remote (i.e., within a different document).

Schema: font-face-uri

        <define name='font-face-uri'>
          <element name='font-face-uri'>
            <ref name='font-face-uri.AT'/>
            <ref name='font-face-uri.CM'/>
          </element>
        </define>

        <define name='font-face-uri.AT' combine='interleave'>
          <ref name='svg.Core.attr'/>
          <ref name='svg.XLinkRequired.attr'/>
          <ref name='svg.External.attr'/>
        </define>

        <define name='font-face-uri.CM'>
          <zeroOrMore>
            <choice>
              <ref name='svg.Desc.group'/>
            </choice>
          </zeroOrMore>
        </define>

Attribute definitions:

xlink:href = "<IRI>"

  An IRI reference to the the font.

  Animatable: no.

Example font03 references an external SVG font, which was defined in example font01.svg.

Example: font03.svg

    <?xml version="1.0" encoding="UTF-8"?>
    <svg version="1.2" baseProfile="tiny" viewBox="0 0 110 70" xmlns="http://www.w3.org/2000/svg"
        xmlns:xlink="http://www.w3.org/1999/xlink">
        <title>Font example</title>
        <defs>
            <font-face font-family="larabie-anglepoise">
                <font-face-src>
                    <font-face-uri xlink:href="font01.svg#la"/>
                </font-face-src>
            </font-face>
        </defs>
        <rect x="00" y="00" width="110" height="70" stroke="#777" fill="none"/>
        <text x="10" y="50" font-family="larabie-anglepoise" font-size="70" fill="#FDD" stroke="#533"
            stroke-width="1.6">SVG</text>
    </svg>