[css-values-4] Add ic unit for ideographic advance.

Author: fantasai

css-values/Overview.bs

@@ -872,7 +872,13 @@ Relative lengths</h3>
 		<tr><td>''ex''
 			<td>x-height of the element's font
 		<tr><td>''ch''
-			<td>width of the "0" (ZERO, U+0030) glyph in the element's font
+			<td>average <a lt="advance measure">character advance</a>
+				of a narrow glyph in the element’s font,
+				as represented by the “0” (ZERO, U+0030) glyph
+		<tr><td>''ic''
+			<td>average <a lt="advance measure">character advance</a>
+				of a fullwidth glyph in the element’s font,
+				as represented by the “水” (CJK water ideograph, U+6C34) glyph
 		<tr><td>''rem''
 			<td>font size of the root element
 		<tr><td>''vw''
@@ -904,7 +910,7 @@ Relative lengths</h3>
 -->
 
 <h4 id="font-relative-lengths">
-Font-relative lengths: the ''em'', ''ex'', ''ch'', ''rem'' units</h4>
+Font-relative lengths: the ''em'', ''ex'', ''ch'', ''ic'', ''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.
@@ -949,11 +955,16 @@ 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
+			Equal to the used <a>advance measure</a> of the “水” (CJK water ideograph, U+6C34) glyph
 			found 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.)
 
+			This measurement is an approximation
+			(and in monospace fonts, an exact measure)
+			of a single narrow glyph’s <a>advance measure</a>,
+			thus allowing measurements based on an expected glyph count.
+
 			Note: The advance measure of a glyph depends on writing-mode and text-orientation
 			as well as font settings, text-transform, and any other properties that affect glyph selection or orientation.
 
@@ -964,6 +975,22 @@ Font-relative lengths: the ''em'', ''ex'', ''ch'', ''rem'' units</h4>
 			(i.e. 'writing-mode' is ''vertical-rl'' or ''vertical-lr''
 			and 'text-orientation' is ''text-orientation/upright'').
 
+		<dt><dfn id="ic" lt="ic">ic unit</dfn>
+		<dd>
+			Equal to the used <a>advance measure</a> of the “0” (ZERO, U+0030) glyph
+			in the font used to render it.
+
+			This measurement is an approximation
+			(and in monospace fonts, an exact measure)
+			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.
+
+			Note: The advance measure of a glyph can depend on writing-mode and text-orientation
+			as well as font settings, text-transform, and any other properties that affect glyph selection or orientation.
+
+			In the cases where it is impossible or impractical to determine the ideographic advance measure,
+			it must be assumed to be 1em.
+
 		<dt><dfn id="rem" lt="rem">rem unit</dfn>
 		<dd>
 			Equal to the computed value of 'font-size' on the root element.
@@ -2228,6 +2255,8 @@ Acknowledgments</h2>
 	of this module.
 
 	Secondly, we would like to acknowledge
+	Koji Ishii
+	and
 	Xidorn Quan
 	for their comments and suggestions,
 	which have improved Level 4.
@@ -2238,7 +2267,7 @@ Changes</h2>
 	Changes since <a href="http://www.w3.org/TR/css-values-3/">Level 3</a>:
 
 	<ul>
-		<li>Added the ''vi'' and ''vb'' units.
+		<li>Added the ''vi'', ''vb'', and ''ic'' units.
 	</ul>
 
 <h2 class="no-num" id="sec-pri">