[css-scroll-snap] Clarify that off-screen snap areas can't create a valid snap position, even if they align as specified by scroll-snap-align.

Author: fantasai

css-scroll-snap/Overview.bs

@@ -529,6 +529,28 @@ Scroll Snapping Alignment: the 'scroll-snap-align' property {#scroll-snap-align}
 			the position applies to both axises.
 	</dl>
 
+	Since the purpose of scroll snapping is to align content within the viewport
+	for optimal viewing:
+	in all cases, the specified alignment creates a valid <a>snap position</a>
+	only if at least part of the <a>snap area</a> is within the <a>snap viewport</a>.
+	For example, a <a>snap area</a> is top-aligned to the <a>snap viewport</a>
+	if its top edge is coincident with the <a>snap viewport</a>’s top edge;
+	however, this alignment is nonetheless not a valid <a>snap position</a>
+	if the entire <a>snap area</a> is outside the <a>snap viewport</a>.
+
+	<details class="why">
+		<summary>Why</summary>
+		As the <a href="https://www.webkit.org/blog/4017/scroll-snapping-with-css-snap-points/">WebKit implementers point out</a>,
+		extending a snap edge infinitely across the canvas
+		only allows for snapping gridded layouts,
+		and produces odd behavior for the user
+		when off-screen elements do not align
+		with on-screen elements.
+		(If this requirement is onerous for implementers however,
+		we can default to a gridded behavior
+		and introduce a switch to get smarter behavior.)
+	</details>
+
 	For all of these values,
 	the <a>block</a> or <a>inline</a> axis
 	is relative to the element's parent's <a>writing mode</a>.