Re: [quicwg/base-drafts] PRIORITY frame on control stream referencing unopened request stream (#2502)

[Patrick Meenan <[e-mail]>]

Getting JavaScript and CSS "right" tends to have more of an impact on the initial page render and app interaction (faster first paint) even though there are fewer of them than images. Interlacing images works well for progressive image formats but not for, say, SVG's (though those tend to be tiny - PNG's on the other hand can be huge).

IMHO, an optimal ordering looks something like this:
- Serialize the CSS and blocking JS in the head
- Load the visible images in parallel (simple round robin)
- Serialize async (and late-body) JS and load them in parallel with non-visible images. The non-visible images might be better to serialize in document order for long-scroll pages instead of loading ALL of them in parallel

If you're going to pick a default, a sequential list works much better than parallel, otherwise you get all of the images stealing bandwidth from the CSS/JS and delaying the initial render. The downside in that case is images loading one at a time instead of concurrently but it's much better than the no-render case.

Discovering when something is "visible" changes over time which is the main reason reprioritization is critical.

As far as late-loading high-priority resources go, Fonts are probably the clearest example since they are not discovered until layout and by the time they are discovered they are needed pretty much immediately to render the text.

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