Our themes use a plenty of CSS3 techniques to achieve an as image-free implementation as possible. Gradients, Text and Block shadows as well as other similar effects are generated primarily using CSS3 and rules.
Nevertheless, we always try to maintain compatibility with browser habits that originate from the past. To achieve this we test our implementations in several browsers and if needed in some cases we may stick with .PNG driven effects.
Regarding the layout and considering the maturity of browsers nowadays, our themes are completely and exclusively based on DIV layouting techniques.
The semantic elements of the HTML5 are under consideration. We are monitoring very close the Drupal development process and we will act accordingly when the platform will be ready for that. We would like to avoid forcing our users to become the pioneers in this area and pay the price of research and experimentation (if any).
The great feature of the client-side database capabilities that HTML5 bring is not needed for our theme implementations. We do use it when it comes to custom projects though. Video and audio tags have been used in several of our projects so far. If we release a theme in the future with a thematology related to video and audio we will base our implementation on that.
In case this article leaves any of your questions or concerns unanswered, please feel welcome to drop us a line at firstname.lastname@example.org.