Label should not allow labelable content except labeled element
Currently <label> accepts any phrasing content but the specification forbids labelable elements (unless it is the element being labeled)
Reduced test-case
The following should fail since the <button> is not the element being labeled:
<label for="foo">
<button>..</button>
</label>
<input id="foo">The following should be allowed since in both cases the <input> field is the labeled element:
<label>
<input>
</label><label for="foo">
<input id="foo">
</label>