On the XF home page there is a <header> tag. I have tried searching html <header> and even just <header> in Google and nothing relevant seems to come up. The validator doesn't like it so it seems to not be html nor php.
What language is <header> and how is it used on the home page?
Note: This forum section seems more like a CSS question area. I can't tell what the best forum is for general HTML questions like this one. Feel free to move the thread as you deem fit.