This feature enables authors to block rendering of a Document until the critical content has been parsed, ensuring a consistent first paint across all browsers. Without this feature, the first paint's state depends on the heuristics for parser yielding which can vary across browsers. This is particularly important for View Transitions where the parsed DOM state on the first frame can drastically change the transition created. Note that this feature specifically implements a `<link rel=expect href="#id">` syntax that allows a link element to reference another expected element on the page. The rendering is then blocked until the expected element is fully parsed. This supersedes previous implementation of html attribute that allows the whole document to be render blocked.
The Web is designed with a model for incremental rendering. When a Document is loading, the browser can render its intermediate states before fetching all the requisite sub-resources, executing all script or fetching/parsing the complete Document. While this is great to reduce the time for first paint, there is a tradeoff between showing a jarring flash of intermediate Document state (which could be unstyled or have more CLS) vs blocking rendering on high priority sub-resources within a reasonable timeout. The render-blocking concept helps browsers in making this tradeoff. It lets authors specify the set of stylesheets and script elements which should block rendering. For example, a stylesheet with the rules necessary to ensure a stable layout. But authors can’t specify which nodes should be added to the DOM before first render. This proposal aims to fill this gap.
Explainers: https://github.com/WICG/view-transitions/blob/main/document-render-blocking.md#blocking-element-id