One thing I didn't mention in the QA post that I still don't know is that without sky setting, Gzweb showed the color of the sky as white/light gray, while with sky it's blacked out, as pictures below. So I assume it's <sky> tag in SDF that's not yet taken by Gzweb but <background> tag is considered (see the commit that is probably making the difference below).
osrf-migration
commented
9 years ago
Original report (archived issue) by Isaac Saito (Bitbucket: 130s).
See http://answers.gazebosim.org/question/8421/no-skyx-texture-on-gzweb/
One thing I didn't mention in the QA post that I still don't know is that without sky setting, Gzweb showed the color of the sky as white/light gray, while with sky it's blacked out, as pictures below. So I assume it's
<sky>tag in SDF that's not yet taken by Gzweb but<background>tag is considered (see the commit that is probably making the difference below).Without tag:
With tag: 
A related ticket in downstream.