[css-values] Let 1ch=.5em in default cases.

Author: tabatkins

css-values/Overview.bs

@@ -598,6 +598,8 @@ Font-relative lengths: the ''em'', ''ex'', ''ch'', ''rem'' units</h4>
 		<dd>
 			Equal to the used advance measure of the "0" (ZERO, U+0030) glyph
 			found in the font used to render it.
+			In the cases where it is impossible or impractical to determine the measure of the "0" glyph,
+			a value of 0.5em must be assumed.
 
 		<dt><dfn title="rem">rem unit</dfn>
 		<dd>

css-values/Overview.html

@@ -64,7 +64,7 @@
 </p>
   <h1 class="p-name no-ref" id=title>CSS Values and Units Module Level 3</h1>
   <h2 class="no-num no-toc no-ref heading settled heading" id=subtitle><span class=content>Editor’s Draft,
-    <span class=dt-updated><span class=value-title title=20140415>15 April 2014</span></span></span></h2>
+    <span class=dt-updated><span class=value-title title=20140418>18 April 2014</span></span></span></h2>
   <div data-fill-with=spec-metadata><dl><dt>This version:<dd><a class=u-url href=http://dev.w3.org/csswg/css-values/>http://dev.w3.org/csswg/css-values/</a><dt>Latest version:<dd><a href=http://www.w3.org/TR/css-values/>http://www.w3.org/TR/css-values/</a><dt>Editor’s Draft:<dd><a href=http://dev.w3.org/csswg/css-values/>http://dev.w3.org/csswg/css-values/</a><dt>Previous Versions:<dd><a href=http://www.w3.org/TR/2013/CR-css3-values-20130730/ rel=previous>http://www.w3.org/TR/2013/CR-css3-values-20130730/</a><dd><a href=http://www.w3.org/TR/2013/CR-css3-values-20130404/ rel=previous>http://www.w3.org/TR/2013/CR-css3-values-20130404/</a><dd><a href=http://www.w3.org/TR/2012/CR-css3-values-20120828/ rel=previous>http://www.w3.org/TR/2012/CR-css3-values-20120828/</a><dd><a href=http://www.w3.org/TR/2012/WD-css3-values-20120308/ rel=previous>http://www.w3.org/TR/2012/WD-css3-values-20120308/</a><dd><a href=http://www.w3.org/TR/2011/WD-css3-values-20110906/ rel=previous>http://www.w3.org/TR/2011/WD-css3-values-20110906/</a><dd><a href=http://www.w3.org/TR/2006/WD-css3-values-20060919 rel=previous>http://www.w3.org/TR/2006/WD-css3-values-20060919</a><dd><a href=http://www.w3.org/TR/2005/WD-css3-values-20050726 rel=previous>http://www.w3.org/TR/2005/WD-css3-values-20050726</a><dd><a href=http://www.w3.org/TR/2001/WD-css3-values-20010713/ rel=previous>http://www.w3.org/TR/2001/WD-css3-values-20010713/</a>
     <dt>Feedback:</dt>
         <dd><a href="mailto:www-style@w3.org?subject=%5Bcss-values%5D%20feedback">www-style@w3.org</a>
@@ -736,6 +736,8 @@ <h4 class="heading settled heading" data-level=5.1.1 id=font-relative-lengths><s
 		<dd>
 			Equal to the used advance measure of the "0" (ZERO, U+0030) glyph
 			found in the font used to render it.
+			In the cases where it is impossible or impractical to determine the measure of the "0" glyph,
+			a value of 0.5em must be assumed.
 
 		<dt><dfn data-dfn-type=dfn data-export="" id=rem title=rem>rem unit<a class=self-link href=#rem></a></dfn>
 		<dd>