[css-flexbox] Preliminary wording for “spatial navigation” vs “sequential navigation” note.

Author: fantasai

css-flexbox/Overview.bs

@@ -1354,6 +1354,15 @@ Reordering and Accessibility</h4>
 		would be non-conformant.
 	</div>
 
+	Note: User agents, including browsers, accessible technology, and extensions,
+	may offer spatial navigation features.
+	This section does not preclude respecting the 'order' property
+	when determining element ordering in such spatial navigation modes;
+	indeed it would need to be considered for such a feature to work.
+	However a UA that uses 'order' in determining sequential navigation,
+	but does not otherwise account for spatial relationships among elements
+	(as expressed by the various layout features of CSS including and not limited to flex layout),
+	is non-conforming.
 
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