Browser vendors have long opposed making backwards incompatible changes. The problem is if any existing websites start breaking some users will switch browsers

Browser vendors have long opposed making backwards incompatible changes. The problem is if any existing websites start breaking some users will switch browsers because of it. Browsers that don't implement the backwards incompatible change will in turn gain users. No browser wants to lose users, all browsers want to gain users, so no browser is willing to make any changes that cause old websites to break. Once it's a browser feature, it's always a browser feature (with very limited exceptions).