[css2] Fix line-height:normal with only fallback fonts

Closes w3c#1800

Author: frivoal

css2/visudet.html

@@ -1415,7 +1415,9 @@ <h3>10.8.1 <a name="leading">Leading and half-leading</a></h3>
 each glyph, determine the <var>A</var> and <var>D</var>. Note that
 glyphs in a single element may come from different fonts and thus need
 not all have the same <var>A</var> and <var>D</var>. If the inline box
-contains no glyphs at all, it is considered to contain a <a
+contains no glyphs at all,
+or if it contains only glyphs from fallback fonts,
+it is considered to contain a <a
 href="#strut">strut</a> (an invisible glyph of zero width) with the
 <var>A</var> and <var>D</var> of the element's first available font.
 

css2/visudet.src

@@ -1319,7 +1319,9 @@ each other by their relevant baselines. Then, for
 each glyph, determine the <var>A</var> and <var>D</var>. Note that
 glyphs in a single element may come from different fonts and thus need
 not all have the same <var>A</var> and <var>D</var>. If the inline box
-contains no glyphs at all, it is considered to contain a <a
+contains no glyphs at all,
+or if it contains only glyphs from fallback fonts,
+it is considered to contain a <a
 href="#strut">strut</a> (an invisible glyph of zero width) with the
 <var>A</var> and <var>D</var> of the element's first available font.