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Priority Hints provide developers a way to indicate a resource's relative fetch priority to the browser, allowing more control over the order resources are loaded.
Many factors influence a resource's priority in browsers. These include type, visibility, and preload status of a resource. Priority Hints introduces a developer-set "fetchpriority" HTML attribute and "priority" fetch property allowing developers to influence the computed priority of a resource fetch. Supported priority values are auto, low, and high.
By far, the biggest win during Origin Trials was for sites to be able to tag important images as high-priority so they could be fetched earlier without having to wait for layout (leading to substantial improvements in LCP). Details are in the linked OT feedback summary.
The only change during the spec process was a name change of the HTML/fetch attributes from "importance" to "fetchpriority" and "priority".