We had this discussion before.
This little perl ditty will test for proper comment syntax:
According to that, and to the validation service, and to ISO8879, and
hence according to the HTML 2.0 spec, yes, that's a legal comment.
(but be careful with the number of -'s! One more or one less, and
it's not a legal comment any more!)
If you limited your support to
<!-- anything but two -'s -->
I'm sure nobody would complain too loudly.
But I think it would only take about 30 minutes of head-scratching to
go ahead and implement the full syntax, and then you'd never have
to worry about it again.