The HTTP 103 status code, also known as “103 Early Hints,” is a provisional response from a web server that helps improve page load performance.
When a server receives a request, it sends a 103 Early Hints response before the final response, allowing browsers to preload critical resources like CSS, JavaScript, or images.
This mechanism enables browsers to start processing and downloading essential assets while the server prepares the main response, resulting in faster page rendering and improved user experience.
What Causes the 103 Early Hints?
The HTTP 103 response occurs when web servers want to optimize resource loading by providing preliminary headers to the browser. This happens in scenarios where the server knows certain resources will be needed but requires time to generate the complete response. Common situations include:
- Complex database queries that take time to process
- Server-side rendering of dynamic content
- Large web applications with multiple dependencies
- Content management systems generating personalized pages
The server uses 103 Early Hints to inform browsers about critical resources through Link headers, enabling parallel processing and reducing overall page load time.
Example
Request:
GET /blog-post HTTP/1.1
Host: example.com
Accept: text/html
Response:
HTTP/1.1 103 Early Hints
Link: </styles/main.css>; rel=preload; as=style
Link: </scripts/app.js>; rel=preload; as=script
HTTP/1.1 200 OK
Content-Type: text/html
[Rest of the response...]
In this example, while the server processes the blog post content, it sends a 103 Early Hints response containing Link headers for the CSS and JavaScript files. The browser can start downloading these resources immediately, even before receiving the main HTML content.
When the final 200 OK response arrives, the browser has already made progress in loading the required resources, resulting in a faster page load experience.
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