H.265 HEVC
The successor to H.264 that halved the bitrate — and then tripped over its own licensing.
HEVC was standardised in 2013 and gives roughly the same quality as H.264 at half the data, which was the precondition for 4K streaming. Technically it is a clear improvement, with larger and more flexible block structures.
The problem was commercial. Instead of one patent pool, three competing ones emerged with different and partly incompatible terms, plus rights holders outside all of them. Uncertainty about what it actually cost made browser vendors abstain, which opened the door for AV1. H.265 nonetheless dominates where the hardware is closed: streaming boxes, surveillance cameras and Apple's ecosystem.