[css3-sizing] improve notes on multi-col sizing

Author: fantasai

css3-sizing/Overview.src.html

@@ -518,7 +518,12 @@ <h3 id="multicol-intrinsic">
 		See also
 		<a href="http://lists.w3.org/Archives/Public/www-style/2011Apr/0316.html">this explanation</a>
 		<!-- http://lists.w3.org/Archives/Public/www-style/2011May/0468.html -->
+
 <!--
+
+	<p class="issue">
+		This section is VERY experimental. Feedback is encouraged. Shipping implementations is not.
+
 	<p>For multi-column elements laid out with infinite available measure (see
 		<a href="#orthogonal-multicol">Multi-column Layout in Orthogonal Flows</a>),
 		the <i>min-content</i> extent is the extent that would result
@@ -549,11 +554,25 @@ <h3 id="multicol-intrinsic">
 		of its contents.
 
 	<p>
-		The <i>max-content measure</i> of a multi-column element is calculated by:
+		The <i>max-content measure</i> of a multi-column element
+		with restrained-height columns (i.e. a specified <i>extent<i> or <i>max-extent</i>)
+		is the <i>measure</i> that would exactly include all of its columns.
+		It may be approximated by:
 	<ul>
 		<li>Laying out the element with column-spanning elements given 'display: none',
 			and taking a measure that includes all the columns.
-		<li>Laying out the element, including column-spanning elements, into that resulting measure.
+		<li>Laying out all of the column-spanning elements into that resulting measure,
+			and taking the resulting extent.
+		<li>Subtracting that resulting extent from the specified restraint,
+			laying out the element without column-spanning elements again into this adjusted extent,
+			and taking the measure of all its columns as its <i>max-content measure</i>.
+	<p>
+		or by some more accurate method.
+
+	<p class="note">
+		This approximation can result in some slack, but avoids overflow in the most common cases,
+		where the balanced height of the columns above spanning elements are approximately equal.
+
 -->
 
 <h2 id='extrinsic-sizing'>