[css-values] Corrections to commit 4d281c098093

Author: fantasai

css-values/Overview.bs

@@ -870,10 +870,12 @@ Font-relative lengths: the ''em'', ''ex'', ''ch'', ''rem'' units</h4>
 			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.
 
-			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.
-			If 'text-orientation' is ''text-orientation/upright'',
-			a value of 1em must instead be assumed.
+			In the cases where it is impossible or impractical to determine the measure of the “0” glyph,
+			it must be assumed to be 0.5em wide by 1em tall.
+			Thus, the ''ch'' unit falls back to ''0.5em'' in the general case,
+			and to ''1em'' when it would be typeset upright
+			(i.e. 'writing-mode' is ''vertical-rl'' or ''vertical-lr''
+			and 'text-orientation' is ''text-orientation/upright'').
 
 		<dt><dfn lt="rem">rem unit</dfn>
 		<dd>