Similar as IETF, W3C has a rather informal liaison management as described here: https://www.w3.org/liaisons/ (table including reference contacts)
The IAB maintains a Coordination Group including mailing list: https://datatracker.ietf.org/iabasg/ietfw3c/about/
Small group meeting between the liaison managers on both side as well as some IAB members usually are organized during in-person meeting, using an W3C github repo for collecting agenda items: https://github.com/w3c/w3c-ietf-coord
W3C hosts a public mailing list for broader discussion if needed: https://lists.w3.org/Archives/Public/public-ietf-w3c/