I think the confusion here is the difference between “HTML TADS”, which is fully supported by QTads, and “TADS Web UI” aka “TADS Web Play”, which isn’t supported at all in any modern interpreter.
I tried my best to explain the difference between HTML TADS and Web UI here:
IFDB’s code for TADS Web UI has never worked since we inherited the site from MJR, but if someone wants to dust it off, I’d be willing to help. The code is in PHP, and lives on in the t3legacy branch of the IFDB Github.
The files all start with t3: t3run, t3servers, t3register, t3ping, t3join, t3files.