Judging from the build log it seems as though it's having trouble downloading an entity of some sort (whatever that is), which is causing xsltproc errors.
Created attachment 212871 [details] Build errors. Detailed build log. Interesting bits are found towards the end where xsltproc is choking. Consistently reproducible for the past couple of weeks.
Looks like the dtd is not only used in tests after all.
This is also happening on 2.28.0.
At the moment this is the only build failure blocking the current stable version of gnome. Any chance it could be escalated?
Fixed in all ebuilds, thanks for reporting.