Question

I want to implement a jQuery plugin that converts a selector to HTML. I understand that the mapping between these two syntaxes is many-to-many, but simple selectors would be easy to covert to HTML.

Example:

$('div#my-id.my-class').create();

might generate:

<div id="my-id" class="my-class" />

EDIT:
The purpose is to create an element if the selector returns an empty set, without having to add redundant code.

var jqs = $('div#my-id');
if(jqs.length===0) jqs = jqs.create();
var elementWillAlwaysBeThere = jqs.get(0);

Answer 1

i know this might not be what you are lokking for. the project zen coding is about using a css like syntax to create dom structurs. so i think lokking at there source is going to help you alot, even thou its not pure css selectores.
div#wrapper>ul#imageSlider>li.image$*3 compiles to:

<div id="wrapper">
    <ul id="imageSlider">
        <li class="image1"></li>
        <li class="image2"></li>
        <li class="image3"></li>
    </ul>
</div>

so if you where to use the jQuery port you can do youre example like
$.zc("div#my-id.my-class") again resulting in

<div id="my-id" class="my-class"></div>

happy source reading