HTTP/2 Server Push allowed the origin server to push subresources (CSS, JS) to the client before the client even requested them. However, in practice, Server Push was fundamentally flawed: (1) The server could not accurately know whether the client already had the resource cached in memory or disk cache, causing massive duplicate downloads and wasted mobile bandwidth; (2) Pushed streams frequently raced against browser parser discoveries, contending with higher-priority critical assets; (3) Cross-origin pushes were severely limited or prohibited by browsers. Chrome officially disabled HTTP/2 Push in 2022. RFC 8297 HTTP 103 Early Hints solves these problems by decoupling server discovery from delivery: while the backend is computing a slow dynamic response (e.g. 300ms database query), the edge proxy sends an informational 103 response containing Link headers (rel=preload, rel=preconnect). The client browser checks its local cache first; if missing, it starts fetching assets immediately during server think time.

In a traditional web request flow, the browser sends an HTTP GET and remains completely idle during server Time to First Byte (TTFB, typically 150ms to 500ms for dynamic rendered pages). Only after receiving the 200 OK and parsing the HTML does the browser discover critical fonts, CSS stylesheets, and hero images. With 103 Early Hints, the edge CDN emits a 103 response within 10ms containing preconnect and preload directives. The browser performs TLS handshakes to font/image CDNs and starts downloading the critical CSS bundle 200ms to 400ms EARLIER, directly shaving hundreds of milliseconds off FCP and LCP metrics.

Legacy was a non-standard browser heuristic that had severe resource leakage issues and lacked granular control over memory, cookies, and network state. The W3C Speculation Rules API provides a declarative JSON-based mechanism (