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>