[css-align] Clarify partial implementations vs invalidity.

Author: fantasai

css-align/Overview.bs

@@ -10,6 +10,7 @@ ED: https://drafts.csswg.org/css-align/
 TR: https://www.w3.org/TR/css-align-3/
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
 Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199
+Previous Version: https://www.w3.org/TR/2017/WD-css-align-3-20170407/
 Previous Version: https://www.w3.org/TR/2017/WD-css-align-3-20170215/
 Previous Version: https://www.w3.org/TR/2016/WD-css-align-3-20160614/
 Previous Version: https://www.w3.org/TR/2016/WD-css-align-3-20160519/
@@ -90,6 +91,25 @@ Values</h3>
 	keyword as their property value. For readability it has not been repeated
 	explicitly.
 
+<h3 id="partial">
+Partial Implementations</h3>
+
+	Since it is expected that support for the features in this module
+	will be deployed in stages corresponding to the various layout models affected,
+	it is hereby clarified that
+	the <a href="https://www.w3.org/TR/CSS/#partial">rules for partial implementations</a>
+	that require treating as invalid any unsupported feature
+	apply to any alignment keyword
+	which is not supported across all layout modules to which it applies
+	for layout models in which the implementation supports the property in general.
+
+	For example,
+	if an implementation supports 'align-self' in [[CSS-GRID-1]] and [[CSS-FLEXBOX-1]],
+	then it must treat ''align-content/start'' as invalid
+	unless it is supported in both grid and flex containers.
+	However if that same implementation does not support 'align-self' for block-level elements at all,
+	then a lack of implementation of ''align-self: start''
+	does not trigger this requirement to treat it as invalid.
 
 <h2 id="overview">
 Overview of Alignment Properties</h2>
@@ -1666,7 +1686,10 @@ Aligning Boxes by Baseline</h3>
 <h2 id="changes">
 Changes</h2>
 
-	Changes since the <a href="https://www.w3.org/TR/2017/WD-css-align-3-20170215/">previous Working Draft</a> include:
+	Changes since the <a href="https://www.w3.org/TR/2017/WD-css-align-3-20170407/">previous Working Draft</a> include:
+	<ul>
+		<li>Adding a section to clarify handling of partial implementations.
+	</ul>
 
 <h2 id="privacy-security">
 Privacy and Security Considerations</h2>