[Ginden]

It's plainly untrue. I worked on ECMAScript proposals and TC39 takes backwards compatibility really seriously.

Browsers too take backwards compatibility very seriously and features stop working almost only if these are serious security holes.

[musicale]

You're right about backward compatibility - and yet so many things break anyway. And many web apps of the past only worked on Internet Explorer (much in the way that modern web apps only work on Chrome.)

However, the thing that broke Tuesday's app on Wednesday was probably a library or framework change.