wikipedia.org:
https://en.wikipedia.org/wiki/HTTP/2

Facility to test:
https://tools.keycdn.com/http2-test
Operating systeem CentOS version 7.4 supports HTTP/2.
For HTTP, suppliers of webbrowsers agreed not to support HTTP/2.

DirectAdmin:
in /etc/httpd/conf/extra/httpd-includes.conf
ProtocolsHonorOrder On
Protocols h2 h2c http/1.1

Plesk:
# plesk bin http2_pref disable
# plesk bin http2_pref enable
(also rewriting cipher setup)

I faced a connection error during payment where the user process did not continue after a minor internet failure. Fix: Payment Service Provider (PSP) Mollie stabilized error handling for their library.