Merge pull request #323 from SebastianZ/issue-322

[css-values] Added 'vi' and 'vb' to heading of section 5.1.2

Author: fantasai

css-values/Overview.bs

@@ -992,7 +992,7 @@ Font-relative lengths: the ''em'', ''ex'', ''ch'', ''rem'' units</h4>
 -->
 
 <h4 id="viewport-relative-lengths">
-Viewport-percentage lengths: the ''vw'', ''vh'', ''vmin'', ''vmax'' units</h4>
+Viewport-percentage lengths: the ''vw'', ''vh'', ''vi'', ''vb'', ''vmin'', ''vmax'' units</h4>
 
 	The <dfn export>viewport-percentage lengths</dfn> are relative to the size of the
 	<a href="https://www.w3.org/TR/CSS21/visudet.html#containing-block-details">initial containing block</a>.