The HTTP 103 status code means a server is sending HTTP headers that are likely to appear in the final response.
The most practical example of this is the server sending
Link HTTP headers. Servers can inform clients beforehand that they will probably need the following resources, so clients could start downloading them right away.
HTTP/2 103 Early Hints Link: </tailbreeze.css>; rel=preload; as=style Link: </voodoo.js>; rel=preload; as=script
Servers can send one or more
103 Early Hints responses:
HTTP/2 103 Early Hints Link: </tailbreeze.js>; rel=preload; as=script
Link headers only serve as hints: the server might or might not include those headers in a final response.
HTTP/2 200 OK Content-Length: 4242 Link: </voodoo.js>; rel=preload; as=script Link: </tailbreeze.css>; rel=preload; as=style Link: </tailbreeze.js>; rel=preload; as=script Link: </tailstorm.js>; rel=preload; as=script
Link HTTP header serves the same purpose as the
<link> HTML tag - informing clients to preemptively fetch resources. HTTP headers can be used in contexts where HTML can’t, such as stylesheets, scripts, and web fonts. Despite that, the
<link> tag is used more often.
<link rel="preload" href="https://example.com/voodoo.js" as="script">
103 Early Hints is an experimental status code, so the support among browsers, HTTP libraries, and web servers is very limited.
Chrome has started an experiment to learn the effectiveness of Early Hints.
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