Include CSS to module specific template
In mymodule.info file, add following line:

stylesheets[all][] = css/styles.css


Then, in html.tpl.php be sure to print scripts and styles, like that:

  &lt;?php print $styles; ?&gt;
  &lt;?php print $scripts; ?&gt;
&lt;/head&gt;


Failing to output these will break many modules, and make Drupal Core to fallback to non-javascript behavior, and you don't want this to happen.