> attribute of INS, DEL, Q, and BLOCKQUOTE in HTML 4.01. No rendering was
> suggested and no importance was specified for CITE, and it has been

Generally the specification of rendering has resulted in elements and
attributes being abused to achieve that particular rendering.

I doubt that most of the above elements will ever be important in the HTML
that most people experience, which is primarily advertising in nature,
nor in the main use within companies, which is as a mechanism for thin
client form applications, and these are the real reasons that browsers
don't support them.