With all the talk about HTML 5 support in Firefox and Safari lately, I have been wondering why it matters in any realistic sense. I haven't read anything directly addressing this issue. I've seen comments saying that you can now use certain HTML 5 features, but I don't understand how you can incorporate features of one specification into another. If I have a a document with an XHTML 1.0 doctype, isn't it invalid to use an element not defined in XHTML 1.0? Would using something like the audio or video tag just be relying on the user's browser to know how to interpret these mixed doctypes? Another angle on this question is: Why does HTML 5 support in newer browsers matter unless IE has the same support? Web developers will always be limited by the weakest link (such as sites still essentially requiring compatibility testing for the ancient IE 6, though things are improving in this area recently). So is there any real value in some browsers supporting HTML 5 until all of them do? Modern browsers supporting HTML 5 doesn't realistically get us any closer to being able to use a real HTML 5 doctype. It all rests on IE. Is there a cause for excitement over this that I'm missing? Or is it just that people are happy to see things "moving in the right direction" in a general way?