HTML5 and CSS3 Themeing with Drupal 7
Drupal 7 wasn't built from scratch to support HTML5 and CSS3. There's a good chance Drupal 8 will ship with integrated support for HTML5, but there's lots of time to wait for that. A key decision in moving the Daymuse site to Drupal 7 was to also move it to HTML5 and CSS3 to support future expansion of services and build a mobile-friendly site.
The difficulty with moving to HTML5 within Drupal is that so many core files output HTML in a way that means pretty deep hooks have to be involved to rebuild the system with new output tags. There's a few Drupal 7 themes that have done great work getting there including AdaptiveTheme and Omega (Sky is another good one but actually based on Adaptive these days). The key difference is that Omega is mobile-first focused and that Omega works hand-in-hand with Context and Delta modules to produce contextual layouts.
The closer relationship to HTML5 Boilerplate makes for a solid reason for Daymuse to support Omega.