[css2] Apply the second half of the percentage height errata

http://www.w3.org/Style/css2-updates/REC-CSS2-20110607-errata.html#s.10.5a

Author: SimonSapin

css2/visudet.html

@@ -901,7 +901,8 @@ <h2>10.5 <a name="the-height-property">Content height</a>: the <a href="visudet.
 href="visuren.html#containing-block">containing block</a>. If the
 height of the containing block is not specified explicitly (i.e., it
 depends on content height), and this element is not absolutely
-positioned, the value computes to 'auto'. A percentage height
+positioned, the used height is calculated as if 'auto' was specified.
+A percentage height
 on the <a href="conform.html#root">root element</a> is relative to the
 <a href="#containing-block-details">initial containing block</a>.
 <span class="note">

css2/visudet.src

@@ -813,7 +813,8 @@ respect to the height of the generated box's <a
 href="visuren.html#containing-block">containing block</a>. If the
 height of the containing block is not specified explicitly (i.e., it
 depends on content height), and this element is not absolutely
-positioned, the value computes to 'auto'. A percentage height
+positioned, the used height is calculated as if 'auto' was specified.
+A percentage height
 on the <a href="conform.html#root">root element</a> is relative to the
 <a href="#containing-block-details">initial containing block</a>.
 <span class="note">