[css-sizing-3] Add a note about potential for textarea/iframe content-sizing in L4. w3c#1771

Author: fantasai

css-sizing-3/Overview.bs

@@ -480,6 +480,14 @@ 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>.
+
 <h3 id="intrinsic-contribution">
 Intrinsic Contributions</h3>