Any way to run obsolete HTML, CSS and mostly JavaScript code? [closed]

My question may sound weird, but I’ve been working on this personal (not commercial) project for years and I suddenly find myself in a position where almost nothing works. I haven’t made any changes to the code for a long time, but now it comes up with “undefined function” or “undefined variable” and lots of weird things in the console. Took me years to build this science/mathematical “app”. It is made to run locally, not for the web.

I’m using a Windows 10 computer for development and an Android 11 or 13 for comfort. I did made like 100 searches, but to no avail.

Any help would be greatly appreciated.

Thanks in advance.