[css-overflow-3] Note issue about aliasing line-clamp

Author: frivoal

css-overflow-3/Overview.bs

@@ -755,6 +755,8 @@ Setting 'max-lines' and 'block-overflow' together: the 'line-clamp' property</h2
 
 	For compatibility with legacy content, UAs that support 'line-clamp' must also support <dfn dfn-type=property>-webkit-line-clamp</dfn> as an alias.
 
+	Issue(2847): It is not obvious that the -webkit- alias can work, due to complicated compatibility constraints.
+
 
 Privacy and Security Considerations {#priv-sec}
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