The subscriptions.php file spits out a widget that is invalid in w3c html5.
The following simple changes on line 584-585 would fix them:
<a name="subscribe-blog"></a> is obsolete, the id attribute on the next line does the same thing in every modern browser. It can just be removed.
<form action="" ... empty actions are not allowed in html5, so changing the action to "." which is valid in both html4 and html5 would fix it.
Thanks for your consideration.