Because the browsers don’t all behave exactly the same and so the developer has to write code that will work in all the browsers that they want to support. There is no guarantee that browsers the developer doesn’t bother supporting will work correctly the website.
Sometimes the website will use really old code and only some browsers are designed to run this old code. Internet Explorer is the general example of a browser that both runs a lot of old code that wouldn’t work in Chrome and for doing things different from other browsers.
Netflix would only stream 1080p HD video in browsers that supported a special feature that only Edge supported.
Apple used to stream their keynotes that only worked in Safari because of the video format there were using only worked in Safari.
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