X-UA-Compatible is no longer breaking validation

Remove statement about X-UA-Compatible is breaking validation.
<del>Instead of removing this, maybe we should just change it to something that informs users about the fact that some validators (e.g.:  <a href="http://html5.validator.nu/?doc=http%3A%2F%2Fdemo.html5boilerplate.com">http://html5.validator.nu</a>) might still complain about the `X-UA-Compatible` value.</del>

[Never mind](http://validator.github.io/validator/) (http://html5.validator.nu probably wasn't updated yet)