[css-position-3] Clarify that HTML form controls are non-replaced. w3c#6789

Author: tabatkins

css-position-3/Overview.bs

@@ -923,6 +923,7 @@ Automatic Sizes of Absolutely-Positioned Boxes</h3>
 		::
 			Its [=automatic size=] is its [=stretch-fit size=].
 
+
 		: Otherwise
 		::
 			Its [=automatic size=] is its [=fit-content size=].
@@ -940,6 +941,10 @@ Automatic Sizes of Absolutely-Positioned Boxes</h3>
 
 	The [=automatic minimum size=] of an absolutely-positioned box is always zero.
 
+	Note: To the extent that form controls can be resized,
+	they are expected to be treated as [=non-replaced=] here.
+	In HTML, all form controls are treated as [=non-replaced=].
+
 <h3 id="abspos-margins">
 Auto Margins of Absolutely-Positioned Boxes</h3>