103 Early Hints

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The HTTP 103 Early Hints information response may be sent by a server while it is still preparing a response, with hints about the resources that the server is expecting the final response will link. This allows a browser to start preloading resources even before the server has prepared and sent that final response.

The early hint response is primarily intended for use with the Link header, which indicates the resources to be loaded. It may also contain a Content-Security-Policy header that is enforced while processing the early hint.

A server might send multiple 103 responses, for example, following a redirect. Browsers only process the first early hint response, and this response must be discarded if the request results in a cross-origin redirect. Preloaded resources from the early hint are effectively pre-pended to the Document's head element, and then followed by the resources loaded in the final response.

Syntax

103 Early Hints

Examples

Basic example

The following 103 early hint response indicates a stylesheet style.css might be preloaded by the final response.

103 Early Hint
Link: </style.css>; rel=preload; as=style

Subsequently the server sends the final response. This includes a link to the stylesheet, which may already have preloaded. 

200 OK
Content-Type: text/html

<!DOCTYPE html>
...
<link rel="stylesheet" rel="preload" href="style.css" />
...

Early hint response with CSP

The following example shows the same early hint response but with a Content-Security-Policy header included.

103 Early Hint
Content-Security-Policy: style-src: self;
Link: </style.css>; rel=preload; as=style

The early response restricts preloading to the same origin as the request. The stylesheet will preload if the origin matches.

The final response might set the CSP to none, as shown below. The stylesheet has already preloaded, but will not be used when rendering the page. 

200 OK
Content-Security-Policy: style-src: none;
Content-Type: text/html

<!DOCTYPE html>
...
<link rel="stylesheet" rel="preload" href="style.css" />
...

Specifications

Specification
HTML Standard
# early-hints

Browser compatibility

BCD tables only load in the browser

See also