This below code is forbidden in XHTML 1.1 strict mode:
<form method="post" action="index">
<input id="_method" name="_method" type="hidden" value="" />
</form>
If you want it to be valid then you need to wrap it with a div (or p, etc..) like:
<form method="post" action="index">
<div>
<input id="_method" name="_method" type="hidden" value="" />
</div>
</form>
How does this make any sense at all?