As a base case, the current HTML emitted by a comment ending in ~~~~ in wikitext (with a sample comment text preceding) is:

<dd>The logo was never really non-free, as far as I can tell, it's been replaced by an identical SVG, which wouldn't change the non-free situation in any case.  But SVG is better than PNG, so go ahead, delete away. <a href="/wiki/User:Cananian" class="mw-redirect" title="User:Cananian">C. Scott Ananian</a> 18:07, 29 March 2007 (UTC)</dd>

The typical signature would also have a link to the talk page, like

<dd>The logo was never really non-free, as far as I can tell, it's been replaced by an identical SVG, which wouldn't change the non-free situation in any case.  But SVG is better than PNG, so go ahead, delete away. <a href="/wiki/User:Cananian" class="mw-redirect" title="User:Cananian">C. Scott Ananian</a> (<a href="/wiki/User_talk:Cananian" title="User talk:Cananian">talk</a>) 18:07, 29 March 2007 (UTC)</dd>

Given how free-form the signature is, I think we should put the creator descriptor around the whole signature part, rather than try to add it to a specific link. There could be multiple links to user/talk/contribs pages, or none (although this would be disallowed by policy on WMF wikis), so something with this structure:

<span class="mw-signature">
    <span creatorAttribute="Cananian">
        <!-- user link --> (<!-- talk page link -->)
    </span>
    <time ...>
        <!-- timestamp -->
    </time>
</span>

So...

<span vocab="http://schema.org" typeof="Comment" class="mw-signature">
    <span rel="creator" resource="/wiki/User:cscott"> <!-- machine readable link to username in span attributes -->
       <!-- But note that everything in this <span> is customizable text, don't try to parse inside the tag -->
       <a href="/wiki/User:cscott" title="User:cscott">C. Scott Ananian</a> (<a href="/wiki/User_talk:cscott" title="User talk:cscott">talk</a>)
     </span>
    <time property="dateCreated" datetime="2007-03-29T18:07Z">18:07, 29 March 2007 (UTC)</time>
</span>

?
(You could also add the userid as an attribute to the <span> if that was thought useful.)

Would this be generated by an extension tag or parser function, or would that HTML be directly saved into the page by the editor? The former approach would presumably be cleaner in terms of the wikitext source code.

Is the sig. really a typeof Comment? (As illustrated in the example at T234966#5575981 above.) Wouldn't the associated text (usually but not necessarily preceding the signature) be the Comment?

I haven't been able to find good existing class for signature blocks specifically.

Is a CSS class on the whole block sufficient, or should we have additional separate classes for the configurable-text and timestamp sub-components? The intent of T27141 is to allow users to remove custom styling, either would do. Are there use cases where people would want to style the components separately?

On the other hand, if the timestamp uses a distinct <time> element, then selectors like ".mw-signature>span" and ".mw-signature>time" should work?

In T234966#5634049, @Jc86035 wrote:

Would this be generated by an extension tag or parser function, or would that HTML be directly saved into the page by the editor? The former approach would presumably be cleaner in terms of the wikitext source code.

Parser function, see parent task T230653.

In T234966#6120342, @Pelagic wrote:

On the other hand, if the timestamp uses a distinct <time> element, then selectors like ".mw-signature>span" and ".mw-signature>time" should work?

Yes, and I think this would be sufficient.