Note which browsers only support 103 Early Hints over HTTP/2 or above

[tunetheweb]

Summary

Chrome and Safari only support 103 Early Hints over HTTP/2 or above.
Firefox supports it over HTTP/1.1 as well though say they might change that - CC: @valenting in case any update here.

This used to be clearly defined in BCD data, but was updated in #21083 and the note about Chrome was removed, and now it only shows a note for Safari under rel=preconnect:

This is incorrect to me. Chrome has similar restrictions to Safari (for both preconnect and preload), and Safari restricts all of 103, and not just preconnects (though admittedly that is the only type Safari supports so they are kind of equivalent).

So I think this restriction should be under the main 103 entry (rather than just rel=preconnect) and should be the same for Chrome and Safari. Alternatively we can list this restriction under all three entries but that seems a little excessive to me.

Note: I work on Chrome and maintain our docs on this.

Test results and supporting details

This is not covered by WPT that only covers HTTP/2 tests but is easy enough to test with a simple HTTP/1 node server like this that returns a preload:

const net = require('net');

const hostname = '127.0.0.1';
const port = 3000;

const server = net.createServer((socket) => {
  socket.on('data', (data) => {
    const request = data.toString();

    const requestLines = request.split('\r\n');

    const requestLine = requestLines[0];
    const [method, url, protocol] = requestLine.split(' ');

    let response;
    if (method === 'GET' && url === '/') {
      response = `HTTP/1.1 103 Early Hints\r\nLink: </assets/css/common.css>;rel=preload;as=style\r\n\r\nHTTP/1.1 200 OK\r\nContent-Type: text/plain\r\n\r\nWelcome to the homepage!\n`;
    } else {
      response = `HTTP/1.1 404 Not Found\r\nContent-Type: text/plain\r\n\r\n404 Not Found\n`;
    }

    // Write the response to the socket
    socket.write(response);

    // End the connection
    socket.end();
  });

  socket.on('error', (err) => {
    console.error(`Socket error: ${err.message}`);
  });
});

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);
});

server.on('error', (err) => {
  console.error(`Server error: ${err.message}`);
});

Chrome shows no preload of the CSS file:

Neither does Safari (though it doiesn't even support preload early hint so this isn't a great test I admit, but more difficult to show preconnect—and ultimately we're not changing Safari's support here, only Chrome's):

But Firefox does:

Related issues

Partial reversion of #21083

[Elchi3]

Thank you @tunetheweb 👍

[valenting]

FYI Firefox still supports processing 103 over H1.1
Starting with Firefox 129 this behaviour also has a pref but it's still enabled by default.

[tunetheweb]

Thanks! Good to know as was curious.