I noticed this in XXP 4 also.
You can choose a websafe font, say Tahoma, which doesn't have the italic variant (at least I don't) and it refuses to apply the effect.
Dreamweaver has no problem applying it, it can be done in Photoshop also. Viewing the variant in a browser doesn't seem to be a problem, you just can't add it in XWD if you don't have the variant installed.
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