HTML5 offers a lot of features and customization options you won’t find in HTML4.01. Why stick to outdated features when you can have the convenience of using HTML5 in your WordPress blog or website? This tutorial covers the things you have to do to convert your HTML4.01 WordPress theme to HTML5. You may find it as a long and tedious process than just using other themes that are built for HTML5 but if you already have created your own brand online, changing a theme is not recommended. Before changing to HTML5, you have to know that despite the features and powerful customization options, it is not yet fully supported on most browsers.
Read the tutorial here: http://skyje.com/how-to-convert-wordpress-theme-to-html5/