No common browsers support HTTP/1.1 trailers. Look at the column “Headers in trailer” in the “Network” tab of browserscope.
- Chrome: No, and won’t fix (bug). Supports H/2 trailers (bug).
- Firefox: No, and I don’t see a ticket in bugzilla for it. Appears to support H/2.
- IE: No
- Edge: No
- Safari: No
- Opera: Old versions only (v10 – 12, removed in 14)
As you’ve discovered, a number of non-browser clients support it.