CSS Containment <https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Contain...> defines ways of optimising a lot of calculation and recalculation, roughly by forbidding various of the harder or slower cases. (It’s approximately just an optimisation hint, though it does change behaviour if you try to break out of the containment you declare. It’s conceivable that in the future browsers could determine whether these optimisations can safely be done without the need for the hint, though that could only help relayout, never initial layout.) Some forms of containment that you can declared definitely allow for parallel layout to work without it being speculative.
> I always thought servers could prelayout the HTML to determine X and Ys and widths for different browser viewport sizes and this metadata would speed up layout because you have a good candidate for layout immediately.
They tried this with <img srcset>, where it can have a lot more value since you can fetch a lower-quality version of the image if you don’t need more pixels. In practice, effective use is somewhere between uncommon and extremely rare, depending on how you want to classify things. Some reconceptualisation of that that integrated with container queries <https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_contain...> might have more potential. But all up, things like media queries, viewport units and calc() make any kind of “serialise the layout” concept extraordinarily difficult to do to any useful degree, and for almost no benefit.
> I always thought servers could prelayout the HTML to determine X and Ys and widths for different browser viewport sizes and this metadata would speed up layout because you have a good candidate for layout immediately.
They tried this with <img srcset>, where it can have a lot more value since you can fetch a lower-quality version of the image if you don’t need more pixels. In practice, effective use is somewhere between uncommon and extremely rare, depending on how you want to classify things. Some reconceptualisation of that that integrated with container queries <https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_contain...> might have more potential. But all up, things like media queries, viewport units and calc() make any kind of “serialise the layout” concept extraordinarily difficult to do to any useful degree, and for almost no benefit.