[webkit-dev] Lazy loading

[Maciej Stachowiak]

> On Oct 28, 2019, at 3:08 PM, Rob Buis <rbuis at igalia.com> wrote:
> 
> On 10/28/19 9:07 PM, Maciej Stachowiak wrote:
> 
>>> Yet another possible task is making lazy loading work for CSS backgrounds, this is implemented in the prototype but I don't think there are many tests for it.
>> Is there a way for content authors to opt in/out (depending on the default), or does this just always follow what the default lazy loading setting is?
> 
> From reading the chromium code, it seems to use a combination of feature policy and the loading attribute to decide whether to enable CSS backgrounds lazy loading or not. Hopefully a chromium developer can confirm.

Per the PR for lazy loading[1], the `loading` attribute applies only to `<img>` and `<iframe>` elements. Most CSS background images are not on either of these specific elements. I can imagine one of the following:

- Chromium has a nonstandard extension beyond what is in the PR to make `loading` a global/superglobal attribute (in which case we probably shouldn’t implement that until spec’d)
- The attribute only applies to CSS backgrounds on specifically the `<img>` and `<iframe>` elements, which would be weirdly limited, and also would be a nonstandard extension to the PR
- CSS backgrounds (and other CSS images?) don’t have an individual override, only the global override via Feature Policy.

This raises two further concerns though:
- Lazy loading for CSS images is not specified in the lazy loading PR[1]. Is it specified elsewhere, or is it a nonstandard Chromium extension?
- I couldn’t find a spec for a lazy loading feature policy either, and the Chrome Platform Status page for it gives the status as “removed”[2] so this too might be a nonstandard thing. Or maybe there is a newer  (Doesn’t look like the feature policy aspect is in your patch though.)

I hope I am not being too nitpicky. I do think this is a great feature. I just want to make sure we’re cautious about the line between implementing standards-track stuff vs copying things from Chromium that are nonstandard or unspecified (so far).

It’s also really worrisome if Chrome implemented semantics for the “loading” attribute beyond what is in the PR, as that will make interop a challenge.

Regards,
Maciej

[1] <https://github.com/whatwg/html/pull/3752>
[2] <https://www.chromestatus.com/feature/5641405942726656>