If you search `#editor-html-source` while in the editor, you'll find the following element.

This is some weird editor hack and not a real textbox but may cause accessibility issues since it has no label. A label should either be provided or it should be hidden (the latter might not be possible since focusable elements shouldn't be ARIA hidden but there might be a way).

Element:

<textarea id="editor-html-source" class="monospaceInput"></textarea>