Better clarify how rem/rlh fall back.

Author: tabatkins

css-values-3/Overview.bs

@@ -915,12 +915,8 @@ Relative lengths</h3>
 <h4 id="font-relative-lengths">
 Font-relative lengths: the ''em'', ''ex'', ''ch'', ''rem'' units</h4>
 
-	Aside from ''rem'' (which refers to the font-size of the root element),
-	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 exception is when they occur in the value of the 'font-size' property itself,
-	in which case they refer to the computed font metrics of the parent element
-	(or the computed font metrics corresponding to the initial values of the 'font' property,
-	if the element has no parent).
+	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>">
 		<dt><dfn id="em" lt="em">em unit</dfn>
@@ -976,10 +972,20 @@ Font-relative lengths: the ''em'', ''ex'', ''ch'', ''rem'' units</h4>
 		<dt><dfn id="rem" lt="rem">rem unit</dfn>
 		<dd>
 			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>.
+
+			If used 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.
 	</dl>
 
+	When used in the value of the 'font-size' 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' property,
+	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' property.
+
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css-values/Overview.bs

@@ -921,17 +921,8 @@ Relative lengths</h3>
 <h4 id="font-relative-lengths">
 Font-relative lengths: the ''em'', ''ex'', ''ch'', ''ic'', ''rem'', ''lh'', ''rlh'' units</h4>
 
-	Aside from ''rem'' (which refers to the font-size of the root element)
-	and ''rlh'' (which refers to the line-height of the root element),
-	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 exception is when they occur in the value of the 'font-size' property itself,
-	in which case they refer to the computed font metrics of the parent element
-	(or the computed font metrics corresponding to the initial values of the 'font' property,
-	if the element has no parent).
-	In addition, if the ''lh'' unit is used in the value of the 'line-height' property,
-	it also refer to the computed metrics on 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 <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>
 		<img src="images/Typography_Line_Terms.svg" 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.">
@@ -1050,6 +1041,16 @@ Font-relative lengths: the ''em'', ''ex'', ''ch'', ''ic'', ''rem'', ''lh'', ''rl
 			the 'max-lines' property shall be used instead.
 	</dl>
 
+	When used in the value of the 'font-size' property
+	(or the 'line-height' property, for the ''lh'' and ''rlh'' units)
+	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' property,
+	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.
+
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