[css-values-4] Define root-relative variants of all font-relative units. w3c#6034

Author: fantasai

css-values-4/Overview.bs

@@ -1591,11 +1591,14 @@ Relative Lengths</h3>
 -->
 
 <h4 id="font-relative-lengths">
-Font-relative Lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''lh'', ''rlh'' units</h4>
+Font-relative Lengths: the ''em'', ''rem'', ''ex'', ''rex'', ''cap'', ''rcap'', ''ch'', ''rch'', ''ic'', ''ric'', ''lh'', ''rlh'' units</h4>
 
 	The <dfn export id="font-relative-length">font-relative lengths</dfn>
-	refer to the font metrics of the element on which they are used--
-	or, in the case of ''rem'' and ''rlh'', the metrics of the root element.
+	refer to the font metrics
+	either of the element on which they are used
+	(for the <dfn>local font-relative lengths</dfn>)
+	or of the root element
+	(for the <dfn>root font-relative lengths</dfn>).
 
 	<figure>
 		<img src="images/Typography_Line_Terms.svg" width=361 height=97 alt="The word 'Sphinx' annotated with various font metrics: ascender height, to the top of the h's serif; cap height, to the visually approximate top of the S; the x height, to the visually approximate top of the x; the baseline, along the bottom of S, h, i, n, and x; and the descender height, to the bottom fo the p.">
@@ -1624,6 +1627,14 @@ Font-relative Lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''l
 				will be 20% greater than the computed font size inherited by <code>h1</code> elements.
 			</div>
 
+		<dt><dfn id="rem" lt="rem">rem unit</dfn>
+		<dd>
+			Equal to the computed value of the ''em'' unit on the root element.
+
+			<wpt>
+			css/css-values/calc-rem-lang.html
+			</wpt>
+
 		<dt><dfn id="ex" lt="ex">ex unit</dfn>
 		<dd>
 			Equal to the used x-height of the <a href="https://www.w3.org/TR/css3-fonts/#first-available-font">first available font</a> [[!CSS3-FONTS]].
@@ -1659,6 +1670,10 @@ Font-relative Lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''l
 			css/css-values/calc-ch-ex-lang.html
 			</wpt>
 
+		<dt><dfn id="rex" lt="rex">rex unit</dfn>
+		<dd>
+			Equal to the value of the ''rex'' unit on the root element.
+
 		<dt><dfn id="cap" lt="cap">cap unit</dfn>
 		<dd>
 			Equal to the used cap-height of the <a href="https://www.w3.org/TR/css3-fonts/#first-available-font">first available font</a> [[!CSS3-FONTS]].
@@ -1674,6 +1689,10 @@ Font-relative Lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''l
 			In the cases where it is impossible or impractical to determine the cap-height,
 			the font's ascent must be used.
 
+		<dt><dfn id="rcap" lt="rcap">rcap unit</dfn>
+		<dd>
+			Equal to the value of the ''rcap'' unit on the root element.
+
 		<dt><dfn id="ch" lt="ch">ch unit</dfn>
 		<dd>
 			Equal to the used <a>advance measure</a> of the “0” (ZERO, U+0030) glyph
@@ -1712,6 +1731,10 @@ Font-relative Lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''l
 			css/css-values/calc-ch-ex-lang.html
 			</wpt>
 
+		<dt><dfn id="rch" lt="rch">rch unit</dfn>
+		<dd>
+			Equal to the value of the ''rch'' unit on the root element.
+
 		<dt><dfn id="ic" lt="ic">ic unit</dfn>
 		<dd>
 			Equal to the used <a>advance measure</a> of the “水” (CJK water ideograph, U+6C34) glyph
@@ -1737,16 +1760,9 @@ Font-relative Lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''l
 			css/css-values/ic-unit-012.html
 			</wpt>
 
-		<dt><dfn id="rem" lt="rem">rem unit</dfn>
+		<dt><dfn id="ric" lt="ric">ric unit</dfn>
 		<dd>
-			Equal to the computed value of 'font-size' on the root element.
-			When specified in the 'font-size' property of the root element,
-			or in a document with no root element,
-			''1rem'' is equal to the initial value of the 'font-size' property.
-
-			<wpt>
-			css/css-values/calc-rem-lang.html
-			</wpt>
+			Equal to the value of the ''ric'' unit on the root element.
 
 		<dt><dfn id="lh" lt="lh">lh unit</dfn>
 		<dd>
@@ -1762,8 +1778,7 @@ Font-relative Lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''l
 
 		<dt><dfn id="rlh" lt="rlh">rlh unit</dfn>
 		<dd>
-			Equal to the computed value of 'line-height' property on the root element,
-			converting ''line-height/normal'' to an absolute length as above.
+			Equal to the value of the ''lh'' unit on the root element.
 
 			Note: Setting the 'height' of an element using either the ''lh'' or the ''rlh'' units
 			does not enable authors to control the actual number of lines in that element.
@@ -1778,44 +1793,46 @@ Font-relative Lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''l
 
 	</dl>
 
-	Issue: We can potentially add more typographic units,
-	like cicero, didot, etc.
-	They're just absolute units,
-	and so can be done with the existing units,
-	but is there enough desire for them
-	(potentially for printing use-cases)
-	that it would be worth adding them?
-	Or should we just wait for Houdini Custom Units?
-
-	Some user-agents allow users to apply additional restrictions to font sizes in a document,
-	such as setting minimum font sizes to ensure readability.
-	When used in the context of an element,
-	these additional restrictions must be applied to the [=used value=] of these properties only;
-	they <em>must not</em> affect the resolution of relative units.
-
-	When used outside the context of an element
-	(such as in <a>media queries</a>),
-	these units refer to the metrics corresponding
-	to the initial values of the 'font' and 'line-height' properties.
-	In this context,
-	the units must apply any additional restrictions to the values,
-	contrary to the normal behavior mentioned above.
-
 	When used in the value of the 'font-size' property on the element they refer to,
-	they resolve against the computed metrics of the parent element--
+	the [=local font-relative lengths=] resolve against the computed metrics of the parent element--
 	or against the computed metrics corresponding to the initial values
 	of the 'font' and 'line-height' properties,
 	if the element has no parent.
-	Additionally,
+	Likewise,
 	when ''lh'' or ''rlh'' units are used
 	in the value of the 'line-height' property on the element they refer to,
 	they resolve against the computed 'line-height' and font metrics of the parent element--
 	or the computed metrics corresponding to the initial values
 	of the 'font' and 'line-height' properties,
 	if the element has no parent.
-	(The other font-relative units continue to resolve against
+	(The other font-relative lengths continue to resolve against
 	the element’s own metrics when used in 'line-height'.)
 
+	When used outside the context of an element
+	(such as in <a>media queries</a>),
+	the [=font-relative lengths=] units refer to the metrics corresponding
+	to the initial values of the 'font' and 'line-height' properties.
+	Similarly, when specified in a document with no root element,
+	the [=root font-relative lengths=] are resolved
+	assuming the initial values of the 'font' and 'line-height' properties..
+
+	Some user-agents allow users to apply additional restrictions to font sizes in a document,
+	such as setting minimum font sizes to ensure readability.
+	Such restrictions must be applied to the [=used value=] of the affected properties only;
+	they <em>must not</em> affect the resolution of [=font-relative lengths=] used in properties.
+	However, in other contexts
+	(such as in [=media queries=]),
+	to the extent that they would impact the used font metrics,
+	such restrictions <em>do</em> affect the resolution of [=font-relative lengths=].
+
+	Note: In general, respecting a user's preferences, like minimum font sizes,
+	is desirable;
+	it's useful for a media query like ''(min-width: 40em)''
+	to use the actual font size the document will be displayed in.
+	However, having these preferences affect font-relative lengths <em>in properties on an element</em>
+	was found to not be Web-compatible;
+	too many pages expect these units to be exact multiples of the specified 'font-size',
+	rather than the <em>actual</em> font-size after applying user preferences.
 
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