Use HTML5 for cross-browser solution?

Use HTML5 for cross-browser solution?

Should I use HTML5 elements (like header, section, etc) when I attempt to
create a cross-browser solution?
I know about some scripts who "add" HTML5 elements in IE explorers, but
that means another HTTP request to the server (I mean, to download the
script) and the most important thing for me is the site's performance.



BTW: If you inspect the code here in StackOverflow, you'll see there's no
HTML5 elements.