[css-sizing] Add percentage sizing section to hold flexbox resolution wrt intrinsic min-sizes.

Author: fantasai

css-sizing/Overview.bs

@@ -541,6 +541,9 @@ Extrinsic Size Determination</h2>
 		<dfn export>Extrinsic sizing</dfn> determines sizes based on the context of an element,
 		without regard for its contents.
 
+<h3 id="fill-available-sizing">
+Fill-available Sizing</h3>
+
 	<p>
 		The inner <a>fill-available inline size</a> of a box is&hellip;
 		<ul>
@@ -565,6 +568,20 @@ Extrinsic Size Determination</h2>
 		<a href="http://lists.w3.org/Archives/Public/www-style/2012Oct/0204.html">Example.</a>
 		Maybe it should stop at each formatting root, or something similar?
 
+<h3 id="percentage-sizing">
+	Percentage Sizing</h3>
+
+	Issue: Write something useful here. Like about when to fall back to auto or whatever.
+
+	In order to prevent cyclic sizing,
+	the ''min-height/auto'' value of 'min-height' and 'max-height'
+	does not factor into the percentage size resolution of the box’s contents.
+	For example, a percentage-height block
+	whose <a>flex item</a> parent has
+	''height: 120em; min-height: auto'' will size itself against ''height: 120em''
+	regardless of the impact that 'min-height' might have
+	on the used size of the <a>flex item</a>.
+
 <h2 class=no-num id="changes">
 Changes</h2>