[css-sizing] Move some edits back from Sizing 4 to Sizing 3, then cut Sizing 4 down to a delta spec.

Author: tabatkins

css-sizing-3/Overview.bs

@@ -80,7 +80,7 @@ Terminology</h2>
 
 		<dt><dfn id="definite" lt="definite|definite size">definite size</dfn>
 		<dd>
-			A size that can be determined without measuring content;
+			A size that can be determined without performing layout;
 			that is, a <<length>>,
 			a measure of text (without consideration of line-wrapping),
 			a size of the <a>initial containing block</a>,
@@ -146,7 +146,7 @@ Auto Box Sizes</h3>
 		: <dfn>stretch-fit inline size</dfn>
 		: <dfn>stretch-fit block size</dfn>
 		:: Roughly, the <a>size</a> a box would take
-			if it filled its <a>available space</a>
+			if it filled the <a>available space</a>
 			in the given axis.
 			(See [[#extrinsic]].)
 
@@ -312,7 +312,7 @@ New Keywords for 'width' and 'height'</h3>
 Containing Floats</h3>
 
 	Note: To ensure that a container sizes itself to contain any descendant floats,
-	make sure it's a <a>formatting context</a>.
+	make sure it's a <a>formatting context</a> root.
 	For some layout modes, such as Grid and Flexbox,
 	this is true automatically.
 	For Block layout, this means using ''display: flow-root;''.
@@ -462,6 +462,28 @@ Stretch-fit Sizing</h3>
 	Percentage Sizing</h3>
 
 	Percentages specify sizing of a box with respect to the box’s <a>containing block</a>.
+	When calculating the <a>intrinsic size contribution</a> of the box,
+	intrinsic sizes are instead treated as ''width/auto''.
+
+	<div class="example">
+		For example, in the following markup:
+
+		<pre class='lang-html'>
+			&lt;div style="width: min-content">
+				&lt;div style="width: 50%;">
+					LOOOOOOOOOOOOOOOOOOOONG
+				&lt;/div>
+			&lt;/div>
+		</pre>
+
+		When calculating the width of the outer div,
+		the inner div is treated as ''width: auto'',
+		so the outer div sets itself to the width of the long word.
+		Since the outer div's width didn't depend on "real" layout,
+		though, it's treated as <a>definite</a>
+		for resolving the inner div,
+		whose width resolves to half that of the outer div.
+	</div>
 
 	Although this may require an additional layout pass to re-resolve percentages in some cases,
 	the ''min-width/auto'', ''min-width/min-content'', ''min-width/max-content'', and ''min-width/fit-content'' values