Contact emails
vmp...@chromium.org, chri...@chromium.org
Explainer
https://docs.google.com/document/d/1VvrEGAjcnto5egR_rnX7PR5ymqWayMNRPYSbCf1bC4I
Summary
We propose to introduce an async attribute on HTMLImageElement and SVGImageElement which would act as a hint to Chrome. This attribute would have two states:
“on”: This indicates that the developer prefers responsiveness and performance over atomic presentation of content.
“off”: This indicates that the developer prefers atomic presentation of content over responsiveness.
Motivation
Currently different browsers deal with deferring images differently. Chrome and Safari by default present content atomically (images and text appear together). Visually this looks correct but can have janky behavior since decoding images delays presentation of non-image content. Edge and Firefox by default have a heuristic that may allow image decodes to be deferred and present content without images. This avoids jank and presents non-image content faster, but can occasionally cause visual flashes or popping in of images that the developer did not intend.
This attribute would give developers a tool specify their intent and preference directly.
Risks
Since this attribute acts as a hint, there is little risk in interoperability. Specifically, if other browsers do not implement it, they would not be necessarily violating the spec. Additionally, since this is an attribute browsers may decide not to parse this value at all with no detriment to either the developer or the user.
If we need to remove this feature, it would also be straight forward in that content may still use the attribute but Chrome can silently ignore it.
Edge: No signals.
Firefox: No signals.
Safari: In development (a version of the attribute is in the Safari developer preview release 40)
Web developers: Positive (AMP team indicated desire to start using the attribute)
Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, Chrome OS, Android, and Android WebView)?
Yes.
Link to entry on the feature dashboard
https://www.chromestatus.com/feature/4897260684967936
Requesting approval to ship?
No.
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I think bikeshedding on the name is not productive on this thread:
- This is an intent to implement. The main content here is that Vlad and Chris are interested in implementing this functionality. If they implemented it under the names “asdf1”, “asdf2”, and “asdf3”, that would be fine, as long as when the time came to ship, they updated to match the agreed-upon standard.
- Getting to an agreed-upon standard is happening; see https://github.com/whatwg/html/issues/1920 . However first we need to resolve a question with other implementers about whether the attribute is tri-state (their proposal) or dual-state (Apple’s proposal). That will undoubtedly affect how it is named, so spending time critiquing their starting names for a tri-state proposal isn’t productive since we don’t even know if we’re going to end up with a tri-state proposal anyway.
I’d encourage people to stop worrying about the names. That’s the least-important part of this intent. I appreciate more substantive feedback Addy gave on integration with priority hints, and encourage people to focus on that level of feedback instead.
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As a developer, I'm struggling to figure out what this actually does. The doc talks of sync vs async, but <img> is already async due to fetching, so when can it be sync?I guess there are cases where the img is in the available images cache, is that what it means?
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Addy - to your point, we've found that deferring both image downloads and image rendering until after the above the fold content is a net positive for products. It would be great if this proposal could look at both.
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