It would be nice to add wiki code inside HTML pages, for example, autofill username in a form field.
Topic on Extension talk:HTMLets
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HTMLets are for static HTML only. Extension:RawMsg might allow something like this, not sure.
In any case: pre-filling form fields with user-specific information has to be done with JavaScript anyway. Remember that the generated HTML for pages gets cached. This means that there can be no user-specific content there.
This post was posted by Peachey88 (Flood), but signed as Duesentrieb.