[css-sizing] Per resolution in #1889 (comment), generalize the notion…

… of which input types are compressible to be those that aren't 'button-like', and define 'button-like'. Relates to #1889.

css-sizing-3/Overview.bs

@@ -1021,7 +1021,17 @@ Compressible Replaced Elements</h2>
 
 	* 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>.
+
+	* <{input}> with any <{input/type}> that is not "button-like";
+		this can vary depending on the UA.
+
+		A type is "button-like" in a particular UA if it displays similar to a <{button}> element,
+		where it can contains actual content that determines the layout of the element.
+		In most UAs, the "button", "reset", "submit", and "color" types are button-like;
+		the "file" type is also partially button-like in some UAs,
+		when it's displayed as a combination of a text input (shrinkable)
+		and a button (button-like, and thus not shrinkable).
+
 	* <{select}>, <{textarea}>, <{progress}>, <{meter}>.
 
 <h2 class=no-num id="changes">