There are much smaller differences in behaviour between ES5 and ES6. These differences may lead to errors in scripts written in ES5 while being interpreted by modern browser (supporting ES6). How does this problem can be addressed?
These differences may lead to errors in scripts written in ES5 while being interpreted as ES6.
No. ES6 was carefully designed to be backwards-compatible. There will be no errors in properly written scripts. Even sloppy mode is still supported.
Should any implementation of an ES6 change really break the internet, it will not be shipped by the browser vendors. Instead they pass this as feedback to the technical committee, discuss solutions, and errata will be published.