They allowed for the inclusion of DRM into HTML5. This DRM is not open-source, can't even be source audited and refused to back down. Such a move caused the EFF (electronic frontier foundation) to resign from the group. Here's the EFF's letter where they outline these issues if you want to give it a read: https://www.eff.org/deeplinks/2017/09/open-letter-w3c-director-ceo-team-and-membership

I could also write about them bowing to Google but the DRM thing annoys me more.