I support updating html.arc in general, particularly to bring it more in line with HTML 5. I think I added support for data attributes at some point, but some things like an async attribute for script tags are still difficult, because it expects all attributes to have a value.
In a perfect world, though, it would use a structure like Racket's html parser, and could be used without namespace pollution.
In a more perfect world (to me), Arc would support XML natively. But I'm probably the only person here who would want to write HTML directly in an Arc script.