A tag like <references group=foo/> will output an ordered list in HTML but this output does not contain any indication of the group name foo in it.
Adding it as an attribute such as <ol data-group="foo" ... would be great, because sometimes it is desirable to apply certain CSS (such as certain @counter-style) to all ref lists that share their group name.
Note that a group name may contain spaces or other special characters (but not quotes) so it cannot be simply added as <ol class="references-foo" ...