CSS containment and content-visibility: fast pages, laid out right

Support for the modern layout features sites use to stay fast: CSS containment and content-visibility. When a page marks a section as off-screen or self-contained to speed itself up, the engine now honors that instead of ignoring it. A section that opts to skip rendering while hidden, the content-visibility technique long feeds, comment threads and long documents use so the browser does not lay out what you cannot see, now correctly leaves its hidden contents out while reserving the right amount of space for them, so the page is laid out correctly and scrolls to the right length. The containment a page can put on a box (size, layout, paint, content, strict) is respected too: a box can size itself without measuring its contents, keep its floats and margins to itself, and clip anything that overflows it, with the placeholder-size hints read as well. The result is that content-heavy pages built with these performance features render the way their authors intended, with nothing bleeding out of a clipped box. Pages that use neither are unchanged. It moves the overall platform score to 70.