point to W3C bugzilla as evidence of past WHATWG/W3C work

[masinter]

it might not be clear to the IETF community the extent of past WHATWG on URL prior to this effort.

https://www.w3.org/Bugs/Public/buglist.cgi?list_id=49773&product=HTML%20WG&product=WebAppsWG&product=WHATWG&short_desc=URL&short_desc_type=casesubstring

(or some better query) could be linked in the problem statement summary as evidence of past work not within IETF, even if most of the bugs listed there don't require coordination.