Is it possible to link to a css stylesheet from within the signature? If I put "{{#css:/wiki/user:myname/signature.css}}" then it doesn't apply the stylesheet and if I do <link rel="stylesheet" href="mysite.com/wiki_style.css"> then it says "html errors" (I assume because stylesheets are only supposed to be referenced inside of <head>. Any help?
Topic on Help talk:Signatures
The facts: The same rules apply as elsewhere in the wikitext of any wiki page. Normal wiki users are not allowed to link off to bring in some other stylesheet. That would be a security issue. You can do inline CSS within HTML tags. See Help:Formatting#HTML tags.
The opinion: Eugh! Why would you want to do this? Custom signatures are silly and annoying, and stylised ones particularly so.