VP9
Google's royalty-free video codec — YouTube's workhorse for a decade, and the rehearsal for AV1.
Google bought On2 Technologies in 2010 and released their codecs freely. VP9 was finished in 2013 and gave roughly the same compression as H.265 but without licence fees, which was the entire point. YouTube moved early, and watching 4K there has long meant VP9 without anyone having to think about it.
Support was uneven: Chrome and Firefox supported it immediately, while Apple held out until 2020. Hardware decoding is in most things from the last ten years. AV1 is the successor and builds on the same codebase, now under the Alliance for Open Media rather than Google alone.