Ask HN: What has good backward compatibility?

Retro game console emulators USB 0.5 and 0.7mm pencils Spoken language dialects Web Browsers PDFs Power outlets FRS radios Metric screw threads

One of these days I want to make, unless someone else does it first, an awesome list that celebrates things that are thoroughly standardized.

Microsoft Windows:

Mathematics. Your new theory need to be compatible with proven theories.

Windows, Common Lisp, PHP? What are you looking for specifically?