[css-align] Specify behavior for UAs that haven't implemented smart-safe alignment.

Author: fantasai

css-align/Overview.bs

@@ -492,6 +492,8 @@ Overflow Alignment: the ''safe'' and ''unsafe'' keywords</h3>
 	Issue: It may not be Web-compatible to implement the “smart” default behavior
 	(though we hope so, and believe it to be likely),
 	so UAs should pass any feedback on this point to the WG.
+	UAs that have not implemented the “smart” default behavior
+	must behave as ''unsafe''.
 
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