It depends on the target bitrate, but from what I've seen at high quality, the difference between HEIF and JPEG is comparable to the difference between H.265 and H.264.

I think your second point is more important though. There are basically two main cases for images. On the one hand, you have images which are an input to some process, like editing or publishing, where people are unwilling to sacrifice any information, lest the errors accumulate. On the other hand, you have images intended for final consumption. By modern standards, those images are already so small that saving 50% isn't usually worth either losing compatibility, or putting in extra effort to offer a fallback.

There are some in between niches of course. Apple has bet on one in iOS with phone photography. That makes sense because many people will have a large volume of images, but they're not professional photographers who need perfect fidelity. I don't know if that's actually worthwhile to most users, though, since they too have to deal with the compatibility issue.

My guess would be AV1, on the other hand this is really the space h265 occupies at the moment so maybe they will be effective at holding on to it.

I fully expect AV1 to become the new 'de facto' video format on the web which is the crown h264 has held for ages. No royalties, better compression and wide hardware support coming will make it a very attractive choice.

And as streaming will also move towards 4k+, given that all the streaming giants (Google, Netflix, Amazon) are directly backing the development of AV1 it would surprise me if they did not phase out HEVC in favour of AV1 for all their 4K+ needs as well (Youtube doesn't support HEVC at all).

I don't think h265 will ever be widespread as h264. I am counting on H266, which has an industry formed alliance MC-IF for patents and they include All most of the original H.263, H.264, H.265 members. ( Missing Qualcomm )

Hopefully they will work out a deal and move things forward.

I still consume nearly all of my content in 720p. The bandwidth requirements usually outstrip my need for fidelity. This is especially true when traveling when I don’t have access to my own broadband. It’s easy to get stuck in a place with slow internet services.

Perhaps AV1 will in the loooong term, but as of today h265 is what every 4K Blu-Ray uses and just about every modern device has a hardware decoder. So that’s gotta count for something.

The problem is 4K are streamed in much higher bitrate, making the video looks better. But in reality, if 1080P had the same bitrate you might not be able to tell the difference. Standard Youtube using VP9 stream 4K at nearly 20Mbps. Most of the best encoded BluRay torrent aren't even using this bitrate, and they are still using x264.

> dav1d seems to perform worse than libaom for me on some videos I tried.

As far as I know, the Firefox buildsystem does not compile dav1d with asm yet. So, that is expected, for now.

The firefox implementation is currently lacking the ASM code optimizations and actually runs worse than the default libaom implementation.

There are grammar checkers that can pick up on those errors. Microsoft Word has one built in (green underline versus red).