Use a class on a DIV on specific content with Omega
You don't mention if it's a particular node TYPE that you're after.  If it is, then please clarify that.  You also don't say what the .mask div actually contains, so I'm going to assume it contains nothing and the content is the background image.  If this is the case then you can just put &lt;div class="mask"&gt;&lt;/div&gt; anywhere.  The position: absolute should mean that it won't dislodge any other content anyway.

I'm also not sure why you need !important after width.  If this class is something you're declaring then it shouldn't get overriden unless YOU want it to.  If you're worried about it then I'd namespace the class instead (ie .mytheme-mask).

EDIT

Put it anywhere, but write it like this:

&lt;?php if (!drupal_is_front_page() &amp;&amp; !$teaser) { ?&gt;
  &lt;div class="mask"&gt;&lt;/div&gt;
&lt;?php };?&gt;


This means that if the page is a content page (not the front page) and not a teaser then place your div in the node.