[css-values-4] Correctly fix cyclic definitions with font-relative units. #2115

Author: fantasai

css-values-3/Overview.bs

@@ -919,7 +919,7 @@ Relative lengths</h3>
 <h4 id="font-relative-lengths">
 Font-relative lengths: the ''em'', ''ex'', ''ch'', ''rem'' 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,
+	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'', the metrics of the root element).
 
 	<dl export dfn-type=value dfn-for="<length>">

css-values-4/Overview.bs

@@ -933,7 +933,8 @@ Relative lengths</h3>
 <h4 id="font-relative-lengths">
 Font-relative lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''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,
+	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.
 
 	<figure>
@@ -1033,22 +1034,24 @@ Font-relative lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''l
 			Equal to the computed value of 'font-size' on the root element.
 			When specified on the 'font-size' property of the root element,
 			the ''rem'' units refer to the property's <em>initial value</em>.
+
 		<dt><dfn id="lh" lt="lh">lh unit</dfn>
 		<dd>
 			Equal to the computed value of the 'line-height' property of the element on which it is used,
-			converted to an absolute length by measuring the height of a line box containing only the strut (See [[CSS2/visudet#strut]]) would have.
+			converting ''line-height/normal'' to an absolute length
+			by using only the metrics of the <a href="https://www.w3.org/TR/css3-fonts/#first-available-font">first available font</a>.
 
 		<dt><dfn id="rlh" lt="rlh">rlh unit</dfn>
 		<dd>
 			Equal to the computed value of 'line-height' property on the root element,
-			converted to an absolute length by measuring the height of a line box containing only the strut (See [[CSS2/visudet#strut]]) would have.
+			converting ''line-height/normal'' to an absolute length as above.
 
-			Note: setting the 'height' of an element using either the ''lh'' or the ''rlh'' units
+			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.
 			These units only enable length calculations based on the theoretical size of an ideal empty line;
 			the size of actual lines boxes may differ based on their content.
 			In cases where an author wants to limit the number of actual lines in an element,
-			the 'max-lines' property shall be used instead.
+			the 'max-lines' property can be used instead.
 	</dl>
 
 	Issue: We can potentially add more typographic units,
@@ -1060,15 +1063,24 @@ Font-relative lengths: the ''em'', ''ex'', ''cap'', ''ch'', ''ic'', ''rem'', ''l
 	that it would be worth adding them?
 	Or should we just wait for Houdini Custom Units?
 
-	When used in the value of the 'font-size' property
-	(and additionally for the ''lh'' and ''rlh'' units in the 'line-height' property)
-	on the element they refer to,
-	these units refer to the computed font metrics of the parent element
-	(or the computed font metrics corresponding to the initial values of the 'font' and 'line-height' properties,
-	if the element has no parent).
 	When used outside the context of an element
 	(such as in <a>media queries</a>),
-	these units refer to the computed font metrics corresponding to the initial values of the 'font' and/or 'line-height' properties.
+	these units refer to the metrics corresponding
+	to the initial values of the 'font' and 'line-height' properties.
+	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--
+	or against the computed metrics corresponding to the initial values
+	of the 'font' and 'line-height' properties,
+	if the element has no parent.
+	Additionally,
+	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 element’s own metrics when used in 'line-height'.)
 
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