Web Standards
In the past, browser capabilities varied wildly. To build a particular feature, you had to use one programming command for one browser and a different command for another browser. Also, a website was oftentimes displayed quite differently with various browsers.
Nowadays, most browsers are quite similar in the supported features and the way they display a given website. This is the result of an ongoing effort of the World Wide Web Consortium (W3C) to standardize the web. The consortium aims to achieve the best possible experience regardless of the used browser and the capabilities of the user (website should be also accessible for disabled users).
A new web feature is first drafted and then goes through a public discussing proccess in multiple stages. Once, the final stage is reached and the feature is well defined browsers are urged to implement the feature. It's in the best interest of browser vendors to build such new features because users might gravitate to using a browser that offers the best experience with the newest features.
However, sometimes browser vendors ignore the public standardization process and implement their own features so that other browsers need time to catch up implementing these features as well. Some people see this critically because a browser implementing a feature that no other browser offers, creates a kind of monopoly. This is against the idea of an open web platform. More about standards on MDN.