If you are using internet explorer you will come across some issues while theming omega sub theme.
The reason :
Internet explorer does not process the HTML 5 tags when it finds it.
to delete , posted in issue
http://drupal.org/node/1315636
For example : this tag ( do not confound with will become . The style associated to it will still be , but the block will have a width and height of 0px , making it impossible to theme ).
Soution :
Use a simple Drupal module which will only insert a script from remy sharp into every page head. This script will activate only on non-compatible version of internet explorer and will "help" internet explorer to render HTML 5 tags.
http://drupal.org/project/remysharp_html5shim
This module or script could be intergrated into omega.
After that , your HTML 5 tags will be displaid with the correct style in internet explorer.