[css-values] Rephrase definitions of ch and ic units to emphasize conceptual definition. w3c#7577

Author: fantasai

css-values-3/Overview.bs

@@ -1115,7 +1115,8 @@ Font-relative Lengths: the ''em'', ''ex'', ''ch'', ''rem'' units</h4>
 
 		<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
+			Represents the typical <a>advance measure</a> of European alphanumeric characters,
+			and defined as the used <a>advance measure</a> of the “0” (ZERO, U+0030) glyph
 			in the font used to render it.
 			(The <dfn dfn>advance measure</dfn> of a glyph is its advance width or height,
 			whichever is in the inline axis of the element.)

css-values-4/Overview.bs

@@ -1765,7 +1765,8 @@ Font-relative Lengths: the ''em'', ''rem'', ''ex'', ''rex'', ''cap'', ''rcap'',
 
 		<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
+			Represents the typical <a>advance measure</a> of European alphanumeric characters,
+			and defined as the used <a>advance measure</a> of the “0” (ZERO, U+0030) glyph
 			in the font used to render it.
 			(The <dfn dfn>advance measure</dfn> of a glyph is its advance width or height,
 			whichever is in the inline axis of the element.)
@@ -1807,10 +1808,11 @@ Font-relative Lengths: the ''em'', ''rem'', ''ex'', ''rex'', ''cap'', ''rcap'',
 
 		<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
+			Represents the typical <a>advance measure</a> of CJK letters,
+			and defined as the used <a>advance measure</a> of the “水” (CJK water ideograph, U+6C34) glyph
 			found in the font used to render it.
 
-			This measurement is a typically an exact measure
+			Note: This measurement is a typically an exact measure
 			(in the few fonts with proportional fullwidth glyphs, an approximation)
 			of a single <a href="http://unicode.org/reports/tr11/#Definitions">fullwidth</a> glyph’s <a>advance measure</a>,
 			thus allowing measurements based on an expected glyph count.