[css-sizing-3] Use ICB as default object size for boxes with an aspect ratio but no size.

Author: fantasai

css-sizing-3/Overview.bs

@@ -849,8 +849,8 @@ Intrinsic Sizes</h3>
 				and likely to be web compatible,
 				but please send feedback to the CSSWG if there are any problems.
 			* Otherwise
-				use a width of ''300px''
-				(height of ''150px'' in <a>vertical writing modes</a>)
+				use an [=inline size=] matching
+				the corresponding dimension of the [=initial containing block=]
 				and calculate the other dimension using the aspect ratio.
 		<dt>For boxes without an intrinsic aspect ratio:
 		<dd>
@@ -1258,6 +1258,15 @@ Compressible Replaced Elements</h2>
 <h2 class=no-num id="changes">
 Changes</h2>
 
+	<p>
+		Major changes since the <a href="https://www.w3.org/TR/2019/WD-css-sizing-3-20190522/">22 May 2019 Working Draft</a> include:
+
+	<ul>
+		<li>...
+		<li>Switched default sizing of an object with an intrinsic aspect ratio to use the ICB size instead of 300px&times;150px.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/4218">Issue 4218</a>)	
+	</ul>
+
 	<p>
 		Major changes since the <a href="https://www.w3.org/TR/2018/WD-css-sizing-3-20180304/">4 March 2018 Working Draft</a> include: