A server may need time to produce an HTML response even though it already knows that the page will require a stylesheet or other critical resource. HTTP 103 Early Hints lets the server send selected Link hints before the final response so a supporting client can begin useful work sooner.
The optimization is optional: the final response still determines the page result.
Understand the response sequence
A simplified exchange looks like this:
HTTP/1.1 103 Early Hints
Link: </app.css>; rel=preload; as=style
HTTP/1.1 200 OK
Content-Type: text/html
<!doctype html>...The 103 is informational, not the final status. A client that ignores it must still receive a correct page from the final response.
Hint only resources that are very likely to be needed
Early hints can move resource discovery earlier, but an unnecessary preload wastes bandwidth and may compete with more important requests. Good candidates are stable, render-critical resources known before HTML generation completes.
Avoid user-specific or speculative resources unless their benefit is well understood.
Keep final HTML authoritative
The document should still contain the normal stylesheet, script, or other resource references required for correctness. Do not make page functionality depend on Early Hints support.
Likewise, treat the final response’s status and headers as authoritative. The server may ultimately redirect, return an error, or choose different content after sending an informational response.
Be careful with redirects and authentication
Sending hints before authorization or routing decisions finish can reveal resource names or cause unnecessary requests. Only emit hints that are safe for the request context and do not disclose sensitive implementation details.
A request that may redirect should not aggressively preload resources specific to a destination that has not been established.
Coordinate with existing preload strategy
103 commonly carries Link headers using relations such as preload. The same resource may also be referenced by final HTML or response headers. Keep attributes such as as, crossorigin, and the resource URL consistent so the browser can reuse work rather than issuing a mismatched request.
Measure actual behavior in target browsers and through the production CDN or reverse proxy. Intermediaries may handle informational responses differently, and support can evolve independently of application code.
Common pitfalls
Hinting too many assets
More preloads are not automatically faster. Prioritization and bandwidth contention can make performance worse.
Treating 103 as a cache directive
Early Hints are an optimization signal, not a replacement for correct cache headers on the final resource responses.
Depending on it for correctness
Clients and intermediaries may ignore informational responses. The final response must remain complete.
Skipping measurement
Server-side render time, network latency, browser scheduling, and CDN behavior determine whether Early Hints help. Compare user-centric performance metrics rather than assuming earlier always means faster.
Use Early Hints as a narrow optimization
HTTP 103 is most valuable when the server knows a small set of critical resources before it can finish generating the document. Keep hints conservative, preserve a fully correct final response, and validate the effect through the same network path real users take.