Also, modern Dev Tools in all the major browsers are getting close to being full blown IDEs in their own right (again). It's got a full REPL one F12 (or Ctrl+Shift+I) press away. (Plus modern syntax highlighting, element inspection, direct CSS and sometimes JS file modification.)
(Pre-Edgmium, Edge's Dev Tools even shared a bunch of code with VS Code and briefly explored hosting a full VS Code experience.)
https://en.wikipedia.org/wiki/WorldWideWeb
Netscape was going to have a built in editor, but they ran out of time, was added in Netscape 3 Gold, and there by default in 4.