True, html5 is not supported in all browsers. There's no "yet" because the issue is older browsers, which nobody can control, and will never be upgraded. So it's a matter of your balancing your interest in using new html5 capabilities versus the scope of customers you reach. If you're not actually using html5 tags, then the only downside I can think of is that the Facebook or Twitter code you've included might fail in older browsers. I suspect it won't, because they'd be getting lots of complaints if it did. Frankly my guess is you're not accomplishing anything at all with your switch to html5 doctype, other than some personal "feel good" about seeing the validator tell you the code is okay.