(Now with UPDATE) Here’s how Technorati, where ultra-strict Tantek actually works, codes its weighted lists of tag terms, where more-popular terms are set in a larger font size:
<a style="font-size:16px;line-height:24px" href="/tag/Humor">Humor</a><a style="font-size:9px;line-height:24px" href="/tag/Humour">Humour</a><a style="font-size:10px;line-height:24px" href="/tag/In The News">In The News</a>
A tad crude, really. Tag ragoût, if not actual tag soup.
Self-evidently, a tag collection of this sort is an unordered list with an assumed default size that is modified by big and small:
<ul>
<li><big><big>Humor</big></big></li>
<li><small>Humour</small></li>
<li><big><big><big>In the News</big></big></big></li>
</ul>
It is of course straightforward to style such a list and its nested elements. Suddenly the HTML you use in your tag listing actually passes for semantic. And this is another example of the ways in which HTML can never be entirely structural.
Update
As I mentioned in a comment chez Müllenweg, weighted lists are a visual graphing system. The best you can do is to hope to graft on some semantics. HTML does not intrinsically provide semantics for visual graphing systems. Hence using the ur-structural em, as Tantek now proposes, is simply adding tits to a bull.
Weighted lists are presentational and my solution remains the best available. Evidence that it is the best can be derived from the fact that nobody implements it.