Ok, I am going to add on here...
HTML5 is basically a superset of all prior versions with some old features deprecated (slated for future elimination):
www.tutorialspoint.com/html5/html5_deprecated_tags.htm
FreeToastHost supports HTML5 and uses it by default for our templates. However, as Brian indicates, HTML5 support is tied to browsers. It is supported by all browsers that FreeToastHost supports (not a coincidence).
I will specifically point out one HTML5 feature that is specific to it and may enable better SEO in the future: HTML5 semantic elements (section, article, aside, header, footer, nav, figure, figcaption, time, mark & main) FTH does not use these currently (we might in the future), but you are certainly welcome to use them in your content.
Keep in mind, though, that since FTH is a template based content management system, you do not control our template, you control your content via our admin console forms. This is why we generally prefer that people use the content editors provided rather than trying to hand-craft HTML content... FTH is not really intended for a lot of hand-crafted HTML content.