[css-fonts-4] Clean up font-style section (somewhat)

r=fantasai

Author: litherum

css-fonts-4/Overview.bs

@@ -601,86 +601,107 @@ Font width: the 'font-stretch!!property' property</h3>
 <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: As specified
-Media: visual
-Animatable: If both "from" and "to" values are "oblique", then yes, as an <<angle>>. Otherwise, no.
-</pre>
+    <pre class="propdef">
+    Name: font-style
+    Value: normal | italic | oblique <<angle>>?
+    Initial: normal
+    Applies to: all elements
+    Inherited: yes
+    Percentages: n/a
+    Computed value: As specified
+    Media: visual
+    Animatable: If both "from" and "to" values are "oblique", then yes, as an <<angle>>. Otherwise, no.
+    </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.  Oblique faces can be simulated by artificially sloping the
-glyphs of the regular face. Compare the artificially sloped renderings of Palatino "a" and Baskerville "N"
-in grey with the actual italic versions:
+    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.
 
-<figure>
-	<img alt="artificial sloping vs. real italics" src="images/realvsfakeitalics.png" >
-	<figcaption>Artificial sloping versus real italics</figcaption>
-</figure>
+    <div class="example">
+        Compare the artificially sloped renderings of Palatino "a" and Baskerville "N" in grey
+        with the actual italic versions:
 
-Values have the following meanings:
+        <figure>
+            <img alt="artificial sloping vs. real italics" src="images/realvsfakeitalics.png" >
+            <figcaption>Artificial sloping versus real italics</figcaption>
+        </figure>
+    </div>
 
-<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".
+    Values have the following meanings:
 
-	<dt><dfn>italic</dfn>
-	<dd>
-		Matches against a font that is labeled as an italic face, or an oblique face if one is not.
+    <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>oblique <<angle>>?</dfn>
-	<dd>
-		Controls matching against an oblique face. The lack of a number represents an angle of "20deg". Values less than -90deg or values greater than 90deg are invalid and are treated as parse errors. (Note that a font might internally provide its own mapping for "oblique", but that mapping within the font is disregarded).
-		Matches against a font that is labeled as an oblique face, or an italic face if one is not.
-</dl>
+        <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.
 
-Some font families might contain only zero or one italic or oblique face, while yet other
-families might contain multiple oblique faces with varying
-angles. 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 the
-<a href="#font-matching-algorithm">font matching section below</a> for
-the precise algorithm). Fractional and negative values are accepted, but values
-less than -90 as well as values greater than 90 are not
-allowed and are treated as parse errors.
-For TrueType / OpenType fonts which use variations, the "slnt"
-variation is used to implement oblique values, and the "ital" variation with a value of "1"
-is used to implement the italic values. The meaning of a
-negative value for "oblique" is to slope the text in the opposite
-direction.
-
-If no italic or oblique face is available, oblique faces can 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. The details
-of the obliquing operation are not explicitly defined.
-
-For the purposes of font matching, User Agents may treat ''italic'' as a synonym for
-''oblique''. For User Agents which treat these values distinctly, synthesis must not
-be performed for ''italic''. All user agents may perform synthesis for ''oblique''.
-
-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.
-
-<!-- resolution on the "undefined" nature of synthetics: https://www.w3.org/2013/06/06-css-minutes.html#item03 -->
-
-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 support for <em>installed font fallback</em>.
+        <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 ''20deg''.
+            (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: 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>