I've got my info from here, look under "Media Decoding": https://libreelec.tv/2023/09/28/rpi5-support/

BCM2712 supports HEVC 4K60 hardware decoding. It no longer supports H264 in hardware. This might sound odd but it removes the RPi4’s 1080p restriction on H264 decoding and the 4K H264 test media we have has played. The big increase in performance from the Quad-Core A76 chip means RPi5 can software decode AV1, H264, VC1, VP9, and more at 1080p with ease. In our testing with YouTube and inputstream.adaptive a surprising amount of 4K media also plays. Optimised (lower refresh-rate and bitrate) 4K30 VP9 is generally fine while more demanding 4K60 VP9 content is not possible; it will play but frames are being dropped.