The web platform is always evolving and expanding. A standards body proposes a new feature or API. If the feature seems useful, browser vendors implement it. If enough vendors implement it interoperably, developers begin to use and depend on the feature, and it secures a permanent place in the web platform. Sometimes browser vendors and web developers take the lead and standards bodies write the official version well after the feature is already a de facto standard. In either case, a new feature has been added to the Web. New browsers support it and old browsers do not. Web developers are pulled between wanting to use powerful new features and wanting their web pages to be usable by the largest number of visitors—even those who are not using the latest browsers.
Sometimes browser vendors differ in their ...