IBM's CICS has web support https://www.ibm.com/docs/en/cics-ts/5.6.0?topic=fundamentals...

It does turn out that 3270 streams and web forms are similar. Transmit a form, wait for the user to fill in the fields on the form, then receive the fields when the user submits it. It is how a mainframe in the 1970s/80s can support 100s of terminals, despite being small by modern standards.