[css-sizing] Add the <textarea> auto-sizing behavior, per resolution in #2141. Closes #2141.

Author: tabatkins

css-sizing-3/Overview.bs

@@ -785,13 +785,15 @@ Intrinsic Sizes</h3>
 	replacing the CSS2 “definition”,
 	such as it is.
 
-	Note: The intrinsic sizing keywords might be re-used
-	in a future level of this module
-	to specify content-based sizes for replaced elements
-	such as <{textarea}> and <{iframe}>
-	whose ''height/auto'' size and intrinsic size contributions
-	are currently arbitrarily fixed.
-	See <a href="https://github.com/w3c/csswg-drafts/issues/1771">discussion</a>.
+	When specified on the HTML <{textarea}> element,
+	the ''width/min-content'' and ''width/max-content'' keywords
+	are calculated as normal for a non-replaced [=block container=],
+	treating its <l spec=html>[=raw value=]</l> as child [=CSS/text nodes=].
+
+	Note: See <a href="https://github.com/w3c/csswg-drafts/issues/1771">discussion about this functionality</a>.
+	This might be extended to <{iframe}> or other content-containing replaced elements,
+	but <{textarea}> is the major use-case
+	and has the least additional complications.
 
 <h3 id="intrinsic-contribution">
 Intrinsic Contributions</h3>