The conversion of strings was wrong for `--foo` where it should become
`-Foo`. Instead, it was consuming both the `--` and then would encounter
a `f`.
This moves the relevant algorithms to DOMStringMap and adds
corresponding spec comments to explain what's going on.
This does make us closer to passing
`html/dom/elements/global-attributes/dataset-delete.html` but
unfortunately it still fails because of #12978
Prior to this fix, the value of `d.dataset['-foo']` wasn't undefined.
However, we now fail the assertion where the attribute should remain,
despite the call to delete.
That's because per `LegacyOverrideBuiltIns` we should be checking the
existence of which properties it can delete, before it actually deletes
the attribute. Right now, we do it regardless if it is a valid property
and thus we incorrectly delete the attribute.
Testing: No change in tests, since the test in question has more
assertions that require more fixes
---------
Signed-off-by: Tim van der Lippe <tvanderlippe@gmail.com>
Signed-off-by: Tim van der Lippe <TimvdLippe@users.noreply.github.com>
