If you were using
<meta http-equiv="X-UA-Compatible" content="IE=edge" /> to force IE to use the most recent rendering engine and disable compatibility mode, you might have discovered that it’s not a valid HTML5 meta tag.
The easiest way to solve it is to use
.htaccess or your Apache config to always send X-UA-Compatible in the HTTP headers. Add the following line to your
Header set X-UA-Compatible "IE=edge"
With that set, you won’t see that pesky compatibility mode again, and your HTML5 will validate.