In its previous implementation, the macro ``checkbox_onoff_reversed`` always
created an ``aria-labelledby`` attribute, even if there was no descriptive tag
with the generated ID (used as the value of the ``aria-labelledby``).
Before this patch, the Nu-HTML-Checker [1] reported 255 issues of this type::
The aria-labelledby attribute must point to an element in the same document. (255)
[1] https://validator.w3.org/nu/
Signed-off-by: Markus <markus@venom.fritz.box>
5ad0214bd4