Your wish is granted! Someone added this a while ago. Sigs are now wrapped in two div tags, one with class pmsig, and the other with class pmsig-ID where ID is the user id of the poster. You can add on-site CSS in your user settings to change how they are displayed.
For example, I have sigs set to a font-size of 2px. "What's the point if you make them so small?" you may well ask. Mostly it's so that if someone accidentally gets their post content wrapped in the sig tags, I can zoom in my browser text and still read it easily, yet keep most sigs small enough that they are easy to ignore.
Are you posting in the right place? Check out Where do I post X? to know for sure.
Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
<h5> <h6> <hr /> <i>
<li> <nbsp> <ol> <p>
<small> <strike> <strong>
<sub> <sup> <table>
<td> <th> <tr> <tt>
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
Want more info? How to link
or How to display code and escape characters
are good places to start.