In 2013, the HDMI Forum released version 2.0 of its audio/video spec. Devices compatible with the standard slowly trickled onto the market, but now they've already fallen behind the curve. The HDMI spec has now been bumped to version 2.0a, which adds support for content with high dynamic range—that is, color formats that allow more leeway between bright and dark hues than usual.
According to the official press release, the HDR update is related to metadata extensions released by the Consumer Electronics Association earlier this year. And, well, that's about it. The new spec doesn't appear to include more bandwidth, additional features, or anything else to set it apart from the 2.0 release. The base spec already offers 18Gbps of bandwidth and support for 4K video at up to 60Hz, so it's hardly in desperate need of an upgrade.
Fortunately, the addition of HDR is probably a bigger deal for televisions than the move to 4K resolutions, because it has the potential to improve image quality in a way folks could appreciate from across the room. Let's hope HDR support spreads to desktop PC displays sooner rather than later.