What’s the best to serve commandline output as HTML

Do You mean > Broadcast a running command line applications' output over the internet so that people can view it using their web browser? I would use some sort of social media livestream with screenshare to get it up and running QaD.

If You just want to send the output (Not Live) of a command line app as a snapshot then probably Pipe-ing | the output to create a .txt file in the website folder of your server would work for that.

If You want to view and control the commandline application. You probably want to look into Socket Programming, or set up an SSH connection, maybe there is a web based telnet client plugin like Putty for browsers?

Do you mean like curl?