[css-fonts-5] Split out size-adjust from metric overrides section #6112 and improve its description #6133.

Author: fantasai

css-fonts-5/Overview.bs

@@ -139,13 +139,43 @@ Issue(5892):
 </dl>
 
 
-
+<h3 id="size-adjust-desc">
+Glyph Size Multiplier:
+the 'size-adjust' descriptor</h3>
+
+	<pre class='descdef'>
+	Name: size-adjust
+	Value: normal | <<percentage>>
+	For: @font-face
+	Initial: normal
+	</pre>
+
+	The 'size-adjust' descriptor defines a multiplier
+	for glyph outlines and metrics associated with this font,
+	to allow the author to harmonize the designs of various fonts
+	when rendered at the same 'font-size'.
+
+	All metrics associated with this font--
+	including glyph advances, baseline tables, and overrides provided by ''@font-face'' descriptors--
+	are scaled by the given percentage,
+	as are the rendered glyph images.
+	Consequently,
+	any units derived from font metrics (such as ''ex'' and ''ch'')
+	are also affected when sourced from this font.
+	However, the [=computed value|computed=] 'font-size'
+	(and thus any units such as ''em'' that derive from it)
+	remains unaffected.
+
+	Note: The 'size-adjust' descriptor functions similarly to
+	the 'font-size-adjust' property,
+	which essentially calculates an adjustment per font by matching ex heights,
+	but likewise does not affect the [=computed value|computed=] 'font-size'.
 
 <!-- should this stay in Fonts 4 or be here in Fonts 5? -->
 
 <h3 id="font-metrics-override-desc">
 Default font metrics overriding:
-the 'ascent-override', 'descent-override', 'line-gap-override', and 'size-adjust' descriptors</h3>
+the 'ascent-override', 'descent-override', and 'line-gap-override' descriptors</h3>
 
 <pre class='descdef'>
 Name: ascent-override
@@ -168,17 +198,9 @@ For: @font-face
 Initial: normal
 </pre>
 
-<pre class='descdef'>
-Name: size-adjust
-Value: normal | <<percentage>>
-For: @font-face
-Initial: normal
-</pre>
-
 The 'ascent-override', 'descent-override', and 'line-gap-override' descriptors define the
 <a spec="CSS-INLINE-3">ascent metric</a>, <a spec="CSS-INLINE-3">descent metric</a> and
-<a spec="CSS-INLINE-3">line gap metric</a> of the font, respectively. The `size-adjust`
-descriptor defines a scalar by which glyph outlines and metrics from this font are multiplied.
+<a spec="CSS-INLINE-3">line gap metric</a> of the font, respectively.
 
 When the descriptor value is 'normal', the corresponding metric value is obtained as-if the
 descriptor was absent from the <code>@font-face</code> block.
@@ -190,27 +212,16 @@ For the 'ascent-override', 'descent-override' and 'line-gap-override' descriptor
 descriptor value is a percentage, the corresponding metric value is resolved as the
 given percentage multiplied by the used font size. Negative values are invalid at parse time.
 
-For 'size-adjust', when the descriptor value is a percentage, all metrics values of this font,
-including glyph advance sizes, are resolved as the given percentage multiplied by the used font
-size. When glyphs are visibly drawn, the visual size of the glyphs is also multiplied by the
-percentage.
-
 None of these descriptors affect the calculation of
 <a href="https://drafts.csswg.org/css-values-4/#font-relative-lengths">Font-relative Lengths</a>.
 None of these descriptors affect the behavior of any of the values of 'line-height' other than
 'normal'. The behavior of <code>line-height: normal</code> may be affected by all of these
-descriptors. 'size-adjust' does not affect the computed font size of any element.
-
-When 'size-adjust' is applied in conjunction with 'ascent-override', 'descent-override',
-or 'line-gap-override' in the same @font-face block, the relevant metrics are multiplied by both
-percentages.
+descriptors.
 
 Note: Using the 'ascent-override', 'descent-override', and 'line-gap-override' descriptors can
-achieve a similar effect as specifying a percentage for 'line-height' property. Using the
-'size-adjust' descriptor can achieve a similar effect as specifying a percentage for the
-'font-size' property.
+achieve a similar effect as specifying a percentage for 'line-height' property.
 
-Note: 'ascent-override', 'descent-override', 'line-gap-override', and 'size-adjust'
+Note: 'ascent-override', 'descent-override', and 'line-gap-override'
 are applied per-font rather than per-element. Multiple fonts may be used
 within an individual element e.g. for characters not supported by the primary font.