Blink are shipping a syntax for poster images for plugins.
As part of that, a discussion developed regarding support for different form factors and different resolutions.
The folks there are keen on adopting the srcset syntax for the job, and I think it makes perfect sense.
A few questions remain:
Should that syntax support w descriptors? I believe it should.
What about sizes? In the current model where poster image are of lower priority, there's probably not a huge perf advantage in including it.
What about <video poster>?
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@yoavweiss: I agree that supporting the srcset syntax works well.
If we support w, are you just gonna assume that the size to resolve against is 100vw? If so, all right, but it seems a little weird. I'd rather support sizes or not support w.
<video poster> would probably be good too. I'll betcha we can just upgrade it in-place, yeah? Technically the syntax space is already swallowed up, I think, but literal spaces in urls and commas at the end of urls are so rare we can probably just invade it and call it a day.
@iandevlin Handling responsive video is interesting. Ideally we'd want to standardize automatic quality-switching, like what YouTube does. That's a pretty big task. I'm okay with biting off just what we can chew for now. ^_^
There is a standard already for automatic quality-switching video. It's called DASH. :) It's possible something more would be useful, but we should reason about it separately and look at its use cases and requirements, not just throw something in and hope the outcome will be good or hold poster hostage until videos are responsive.
I don't mind supporting srcset syntax in video poster, if web compat allows. I think x-only at first since currently videos are fixed size.
In httparchive, 1942 pages use <video poster>, 3 pages have spaces in the URL. 0 have comma at the start or end. So it will break a non-zero amount of pages, but it seems OK since a poster image isn't critical, and it's ~0.00% of the pages in the data.