Please give back webkit2gtk on arch:all on stretch-backports. Looking at the test history and comparing with other arches, I think there is a good chance it will build this time. I think the Stable Release Team wants to see this version build on all architectures before approving the stretch-pu bug I'll be filing shortly.
gb webkit2gtk_2.18.6-1~bpo9+1 . all Thanks, Jeremy Bicha