The HDMI Forum released their specifications for HDMI 2.1 earlier this year, but have now tightened them and make them official. HDMI 2.1 is a freaking beast, and will power the next generation of monitors.
HDMI 2.1 packs a huge increase in bandwidth with up to 48Gbps on tap, and is backwards compatible with previous-gen HDMI cables. HDMI 2.1 supports an insane 10K resolution which throws the pixels up to a huge 10240 x 4320, up from the 7680 x 4320 that 8K is capable of. HDMI 2.1 supports higher refresh rates as well, with 4K120 and 8K60 support, as well as Dynamic HDR.
The inclusion of support for 10K is interesting, as it's mainly for commercial and speciality use - but now I want to see a 10K UltraWide monitor.
HDMI 2.1 also supports Variable Refresh Rate, which results in smoother performance on-screen as well as Quick Frame Transport (QFT) which reduces latency. There's also Quick Media Switching (QMS) as well as Auto Low Latency Mode (ALLM) which will automatically set the perfect latency for the smoothest experience.