[CSS] Alias -webkit-appearance for compatibility reasons

Since this is supported as an alias by Firefox and Edge for the same
reasons and we have websites using this to (attempt to) override the
system-provided styling with their own, leaving out the only supported
keyword we'd otherwise have (with -moz- prefix) but still stating
-webkit-.

TODO: unprefix this completely and make the vendor prefixes aliases.

layout/style/nsCSSPropAliasList.h

@@ -264,6 +264,11 @@ CSS_PROP_ALIAS(-webkit-animation-timing-function,
                WebkitAnimationTimingFunction,
                WEBKIT_PREFIX_PREF)
 
+CSS_PROP_ALIAS(-webkit-appearance,
+               appearance,
+               WebkitAppearance,
+               WEBKIT_PREFIX_PREF)
+
 CSS_PROP_ALIAS(-webkit-filter,
                filter,
                WebkitFilter,