It called progressive enhancement. You test whether you have a feature and then code around it if you don't.

e.g. When createObjectUrl was a new thing. I ended up doing something using AJAX and doing some processing server side for IE and Edge. Now I could have done all the processing server-side. However 80%+ of the users use Chrome. So instead of checking everything on the server, I could offload 80% of that to the browser.