[css-sizing-3] Adjust heading level, improve cross-reference.

Author: fantasai

css-sizing-3/Overview.bs

@@ -1242,12 +1242,13 @@ Intrinsic Contributions of Percentage-Sized Boxes</h4>
 		overflowing the <code>&lt;article></code>
 	</div>
 
-<h2 class=no-num id="min-content-zero">
-Compressible Replaced Elements</h2>
+<h4 id="min-content-zero">
+Compressible Replaced Elements</h4>
 
 	The following elements can have their <a>min-content size</a> compressed
 	when their 'width'/'height' or 'max-width'/'max-height' is expressed with a percentage size.
-	See [[#intrinsic-contribution]].
+	See <a href="#replaced-percentage-min-contribution">percentage-sized replaced element rule</a>
+	in [[#intrinsic-contribution]].
 
 	* All <a>replaced elements</a> that aren't form controls.
 		See <a href="https://html.spec.whatwg.org/multipage/rendering.html#replaced-elements">HTML&sect;14.4</a>. [[!HTML]]