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The base HTTP/3 specification doesn't define prioritization itself. The HTTP/2 approach to prioritization has been deemed too complicated and is deprecated by the latest revision of the HTTP/2 specification RFC 9113. A simpler alternative, the Extensible Prioritization Scheme for HTTP (RFC 9218), has been published separately, which can be used in both HTTP/2 and HTTP/3.
In HTTP/3, priority signals are sent in the form of the Priority header field and/or the PRIORITY_UPDATE frame sent on the HTTP/3 Control Stream. Servers can act on these signals to schedule sending of HTTP response content. RFC 9218 provides scheduling guidance that is intended to improve client web application performance.