[css-scroll-snap] Define scroll snap container, use it.

Author: tabatkins

css-scroll-snap/Overview.bs

@@ -219,7 +219,8 @@ Scroll Snapping Rules: the 'scroll-snap-type' property {#snap-type}
 	</pre>
 
 	The 'scroll-snap-type' property is used to specify
-	whether and how strictly <a>snap positions</a> are intepreted on the <a>scroll container</a>,
+	whether a <a>scroll container</a> is a <a>scroll snap container</a>,
+	how strictly <a>snap positions</a> are intepreted on the <a>scroll container</a>,
 	and which axes are considered.
 
 	Issue: <a href="https://lists.w3.org/Archives/Public/www-style/2015Nov/0328.html">We're considering splitting this into subproperties.</a>
@@ -295,6 +296,16 @@ Scroll Snapping Rules: the 'scroll-snap-type' property {#snap-type}
 			the UA must re-snap to it.
 	</dl>
 
+	A box <dfn export>captures snap positions</dfn>
+	if it is a <a>scroll container</a>
+	or has a value other than ''scroll-snap-type/none'' for 'scroll-snap-type'.
+	If a box's nearest <a lt="captures snap positions">snap-position capturing</a> ancestor
+	on its containing block chain
+	is a <a>scroll container</a> with a non-''scroll-snap-type/none'' value for 'scroll-snap-type',
+	that is the box's <dfn export local-lt="snap container">scroll snap container</dfn>.
+	Otherwise, the box has no <a>scroll snap container</a>
+	and its <a>snap positions</a> do not cause snapping.
+
 	Advisement:
 	Authors should use mandatory snap positions with consideration of
 	varyingly-sized screens and (if applicable) varying-sized content.
@@ -489,7 +500,7 @@ Scroll Snapping Alignment: the 'scroll-snap-align' property {#scroll-snap-alignm
 
 	Specifies the element's <a>snap position</a> as an alignment of
 	its <a>snap area</a> (as the <a>alignment subject</a>)
-	within the viewport's <a>snap viewport</a> (as the <a>alignment container</a>).
+	within its <a>snap container's</a> <a>snap viewport</a> (as the <a>alignment container</a>).
 	The two values specify the snapping behavior in the <a>inline axis</a> and <a>block axis</a>,
 	respectively.
 	If only one value is specified,