The ability to composite a portion of a video element over a separate background is widely utilized in video production and broadcasting. This is usually facilitated by pro-oriented mezzanine formats that support transparency (alpha channels). The same techniques are useful in consumer scenarios—such as putting video on a complex background in a presentation or a web page—but common distribution formats do not have support for transparency. To enable these consumer workflows at distribution-friendly bit rates, an alpha channel can now compatibly be added to HEVC using the technique described in this document.
Due to the dominance of Internet Explorer 6, it took a long time before websites could comfortably use PNG files with 8-bit alpha. Now we take it for granted. And yet, we still use the 30+ year old GIF format. Hopefully it won’t take that long with HEVC.