103 Early Hints
Returns response headers before the final HTTP response, primarily used for link preloading.
Usually happens because:
- ☑ Page speed optimization protocols requesting
- ☑ Proxy server configurations forwarding link
🔍 Quick Checklist:
Meaning
The HTTP 103 Early Hints informational status code is primarily intended to be used with the Link header to allow the user agent to start preloading critical resources (like stylesheets or scripts) while the server is still preparing the main page response.
Root Causes
- Page speed optimization protocols requesting asset preloads.
- Proxy server configurations forwarding link headers ahead of payload compilation.
| Cause | Frequency |
|---|---|
| Page speed optimization protocols requesting | ⭐⭐⭐⭐⭐ |
| Proxy server configurations forwarding link | ⭐⭐⭐⭐ |
Common Mistakes
- Using Early Hints on HTTP/1.0 clients that do not parse multiple responses.
- Preloading non-critical resources, resulting in resource bandwidth conflicts.
How to Fix
Framework-Specific Examples
Flushing early hints headers manually in Node.js server connections.
app.get('/', (req, res) => {
res.write('HTTP/1.1 103 Early Hints\r\nLink: </css/main.css>; rel=preload; as=style\r\n\r\n');
res.send('Done');
});Server Configuration Examples
Enabling early hints forwarding inside proxy configurations.
proxy_pass_header Link;Prevention
- Only preload resources that block rendering (e.g. key CSS/Fonts).
- Verify client request headers before flushing early hints.
Frequently Asked Questions (FAQ)
Q: Which browsers support 103 Early Hints?
All modern Chromium-based browsers (Chrome, Edge, Opera) support 103 Early Hints. Safari and Firefox are still evaluating implementation.
Q: How does 103 Early Hints improve page load speed?
It informs the browser of critical CSS/JS dependencies immediately, enabling parallel fetches while the server continues compiling HTML.
Q: Is 103 Early Hints the same as HTTP/2 Server Push?
No. Server Push forces assets onto the client directly. Early Hints tells the client to request assets itself, letting the client check its cache first.
Q: Can I preload image files using 103?
Yes, you can preload critical images (such as LCP hero images) by specifying 'as=image' inside the link header.