All the Liferay themes automatically support html5, its the browsers you should be worried about.
There is nothing special to make a theme HTML5 since its just a matter of doctype change (<!DOCTYPE html>
) to make the browser know that you are rendering HTML5 stuff. So you can check the doctype in portal-normal.vm
and other template files to see if the theme is HTML5.
But for scripting support I doubt if alloy-ui (Liferay's default library) provides any additional HTML5 specific modules for manipulating canvas, file-api etc.