[css-flexbox] Clarified that 'perform layout' means using block-level rules. Fixes #5188.

Author: tabatkins

css-flexbox-1/Overview.bs

@@ -2663,7 +2663,8 @@ Cross Size Determination</h3>
 	<ol class=continue>
 		<li id='algo-cross-item'>
 			<strong>Determine the <dfn>hypothetical cross size</dfn> of each item</strong>
-			by performing layout with the used <a>main size</a> and the available space,
+			by performing layout as if it were an [=in-flow=] [=block-level box=]
+			with the used [=main size=] and the given available space,
 			treating <a value for=width>auto</a> as ''fit-content''.
 
 		<li id='algo-cross-line'>
@@ -3721,6 +3722,16 @@ Changes since the <a href="https://www.w3.org/TR/2018/CR-css-flexbox-1-20181119/
 		<li id="change-2018-flexible-length-text">
 			Slightly restructured the prose in [[#resolve-flexible-lengths]]
 			to resolve <a href="https://github.com/w3c/csswg-drafts/issues/5179">Issue 5179</a>.
+
+		<li id="change-2018-cross-layout">
+			Clarified that “performing layout” means using the block-level layout rules.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/5188">Issue 5188</a>)
+
+			<blockquote>
+				<p><strong>Determine the <dfn>hypothetical cross size</dfn> of each item</strong>
+				by performing layout <ins>as if it were an [=in-flow=] [=block-level box=]
+				with the used <a>main size</a> and the <ins>given</ins> available space &hellip;
+			</blockquote>
 	</ul>
 
 <h3 id="changes-20171016">