[css-values] Per WG resolution, added vi and vb units. For #113.

css-values/Overview.bs

@@ -889,6 +889,10 @@ Relative lengths</h3>
 			<td>1% of viewport's width
 		<tr><td>''vh''
 			<td>1% of viewport's height
+		<tr><td>''vi''
+			<td>1% of viewport's size in the root element's <a>inline axis</a>
+		<tr><td>''vb''
+			<td>1% of viewport's size in the root element's <a>block axis</a>
 		<tr><td>''vmin''
 			<td>1% of viewport's smaller dimension
 		<tr><td>''vmax''
@@ -1018,6 +1022,16 @@ Viewport-percentage lengths: the ''vw'', ''vh'', ''vmin'', ''vmax'' units</h4>
 		<dd>
 			Equal to 1% of the height of the initial containing block.
 
+		<dt><dfn lt=vi>vi unit</dfn>
+		<dd>
+			Equal to 1% of the size of the initial containing block
+			in the direction of the root element's inline axis.
+
+		<dt><dfn lt=vb>vb unit</dfn>
+		<dd>
+			Equal to 1% of the size of the initial containing block
+			in the direction of the root element's block axis.
+
 		<dt><dfn id="vmin" lt="vmin">vmin unit</dfn>
 		<dd>
 			Equal to the smaller of ''vw'' or ''vh''.
@@ -1027,6 +1041,11 @@ Viewport-percentage lengths: the ''vw'', ''vh'', ''vmin'', ''vmax'' units</h4>
 			Equal to the larger of ''vw'' or ''vh''.
 	</dl>
 
+	Issue: All units need to be possible to evaluate in a context without any elements or styling;
+	for example, ''rem'' in a media query is evaluated
+	relative to the document's initial font size.
+	How do we get an inline/block direction from the document?
+
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@@ -2242,6 +2261,7 @@ Changes</h2>
 		<li>Added definition of <<ident>> notation to definition of <a>identifiers</a>.
 		<li>Added <<length-percentage>> as a shorthand for <<length>> | <<percentage>>,
 			along with equivalent productions for angles, numbers, times, and frequencies.
+		<li>Added the ''vi'' and ''vb'' units.
 		<li>Various clarifications and editorial improvements.
 	</ul>