One of my current favorite sites to check see the status of what works with what browser is caniuse.com; It has what most people would consider a complete list of platforms. I am mentioning the site in glowing terms but I need to point out that it is not complete. HTML is used in windows help files and many desktop gadgets and for email. CanIUse is a really great resource but when the conversation starts to talk about desktop applications which get feeds from online new compatibility issues show up.
xHTML vs HTML5 Although there are many html documents that were created as xHTML most are served as HTML documents instead, (by web servers), do to the number of errors that these pages typically have. Because web pages have been live in the past, do not assume they are error free or are really xHTML documents.
Following browser usage:
IE8 usage falls below 10% in the beginning of 2013. as of the date of this post the majority of IE users are on IE 10. But people that have Win XP can only get to IE8 or they need to use a different browser other than Internet Explorer.
IE 6 usage is almost none existent … coding for IE 4-6 compatibility may only need to be done for desktop applications and email these days.