remove id attrs, optionally

[stevenvachon]

…if there is no corresponding selector in the stylesheet.

Before:

<style>
#exists { prop:value }
</style>
<div id="exists"></div>
<div id="non-existent"></div>

After:

<style>
#xz { prop:value }
</style>
<div id="xz"></div>
<div></div>