Plugin Stylesheet 404
Is your css file located in the /resources/ folder of your plugin? So your path should be something like:

&lt;plugin folder&gt;/resources/css/sitemaphtml.css


I've tried it myself and whilst you do get the /cpresources/ part of the Url in your markup, the CSS file is loaded on the front end.

EDIT: If you take a look in the docs, it says the following:


  By default, Craft will search for the resource file in its own
  craft/app/resources/ folder. If it can’t find the file there, it will
  check if the first segment of the resource path is set to a plugin
  handle. If it is, Craft will look for the resource file within that
  plugin’s resources/ folder.