[css-sizing] Define which HTML elements are 'replaced' and thus compressible. Fixes #1889.

Author: tabatkins

css-sizing-3/Overview.bs

@@ -452,6 +452,10 @@ Intrinsic Contributions</h3>
 	However, in the case of a [=replaced element|replaced=] box with a percentage-based 'width'/'max-width'/'height'/'max-height',
 	the percentage is resolved to zero
 	when calculating the <a>min-content contribution</a> in the corresponding axis.
+	(See [[#min-content-zero]] for a list of which elements in HTML this applies to.)
+	The UA may additionally floor this size based on UI considerations,
+	such as ensuring certain UI elements remain visible
+	(for example, the dropdown arrow on a <{select}>).
 
 	<div class=example>
 		For example,
@@ -644,6 +648,18 @@ Extrinsic Size Determination</h2>
 		overflowing the <code>&lt;article></code>
 	</div>
 
+<h2 class=no-num id="min-content-zero">
+Compressible Replaced Elements</h2>
+
+	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]].
+
+	* 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]]
+	* <{input}> with any <{input/type}> other than <code>"button"</code>, <code>"submit"</code>, or <code>"reset"</code>.
+	* <{select}>, <{textarea}>, <{progress}>, <{meter}>.
+
 <h2 class=no-num id="changes">
 Changes</h2>