We can use HTML5 and CSS3 now. Read more about facts and myths here:
There are several tools you can use to make HTML5 work with old browsers.
HTML5 Boilerplate – compilation of the best practices in making HTML5 sites. Pick and choose what's relevant for your project.
Modernizr – to create new HTML5 tags in IEs and detect browser features for graceful degradation.
PIE – use CSS3 features in IE6-8
If you need HTML5 overrides for Joomla core components/modules and K2, there is a Template Framework out there who has done it already.