> See, when they invented new web browsers, the old websites still worked on them!
Yeah. But only because they rendered the old websites in various quirks modes[1] that emulated the rendering engines of the old web browsers.
Thankfully, web browser developers did not share the analagous mindset to the people saying 'lets stick with python2-compatible semantics forever', or we'd still be writing sites that had to be backwards-compatible with IE6, instead of having the option to specify a modern doctype and use IE6-incompatible markup (the analogy to using a python3 shebang line and writing python2-incompatible code).
[1] https://en.wikipedia.org/wiki/Quirks_mode