[css-flexbox] Rewrote container cross-sizing to be generic to the for…

…matting context, rather than assuming block layout. #5190.

css-flexbox-1/Overview.bs

@@ -2803,21 +2803,10 @@ Cross-Axis Alignment</h3>
 			if neither of the item's cross-axis margins are <a value for=margin>auto</a>.
 
 		<li id='algo-cross-container'>
-			<strong>Determine the flex container's used cross size:</strong>
-
-			<ul>
-				<li>
-					If the cross size property is a <a>definite</a> size,
-					use that,
-					clamped by the used min and max <a>cross sizes</a>
-					of the <a>flex container</a>.
-
-				<li>
-					Otherwise,
-					use the sum of the flex lines' cross sizes,
-					clamped by the used min and max <a>cross sizes</a>
-					of the <a>flex container</a>.
-			</ul>
+			<strong>Determine the flex container's used cross size</strong>
+			using the rules of the [=formatting context=] in which it participates.
+			If a content-based [=cross size=] is needed,
+			use the sum of the [=flex lines=]' [=cross sizes=].
 
 		<li id='algo-line-align'>
 			<strong>Align all flex lines</strong> per 'align-content'.
@@ -3732,6 +3721,35 @@ Changes since the <a href="https://www.w3.org/TR/2018/CR-css-flexbox-1-20181119/
 				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>
+
+		<li id="change-2018-cross-sizing">
+			Rewrote flex-contain cross-sizing step
+			to depend on the rules of whatever formatting context it's in,
+			rather than assuming block-layout rules.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/5190">Issue 5190</a>)
+
+			<blockquote>
+				<strong>Determine the flex container's used cross size</strong>
+				<ins>using the rules of the [=formatting context=] in which it participates.
+				If a content-based [=cross size=] is needed,
+				use the sum of the [=flex lines=]' [=cross sizes=].</ins>
+
+				<del>
+					<ul>
+						<li>
+							If the cross size property is a <a>definite</a> size,
+							use that,
+							clamped by the used min and max <a>cross sizes</a>
+							of the <a>flex container</a>.
+
+						<li>
+							Otherwise,
+							use the sum of the flex lines' cross sizes,
+							clamped by the used min and max <a>cross sizes</a>
+							of the <a>flex container</a>.
+					</ul>
+				</del>
+			</blockquote>
 	</ul>
 
 <h3 id="changes-20171016">