to leave the closing </a> away
That's definitely not a good idea.
Another point, about the <dl> variant: one side benefit would be that
<dl> had major problems with formatting, etc. We can't go that route.
I can't fix x-mosaic. I certainly want to make texinfo html output be
as broadly readable as possible, but not at the expense of inaccurate
markup or wrong output.
<a name="..."></a> is the logical way to represent an anchor, and so far
none of the proposed alternatives are acceptable. So I'm afraid we're
all stuck. I hope you can get x-mosaic fixed.