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When the browser can't take a (preconnect) hint

Michael Crenshaw
・4 min read

Resource hints help pages load faster by telling the browser what assets it'll need in the future. For example, a preload hint for a font file tells the browser to start downloading the font before stylesheets are parsed.

<head>
  <!-- other stuff -->
  <link rel="preload" href="https://cdn.example.com/font.woff2" as="font" type="font/woff2" crossorigin>
  <link rel="stylesheet" href="https://cdn.example.com/styles.css" type="text/css">
</head>

Sometimes you don't know ahead of time what assets are needed in a page. But you know they'll be hosted on a particular domain. In that case, you can give the browser a head start with the preconnect resource hint.

<head>
  <!-- other stuff -->
  <link rel="preconnect" href="https://cdn.example.com/" crossorigin>
</head>

Without resource hints

Suppose a page on my site uses a stylesheet on another origin, and that stylesheet defines a custom font.

@font-face {
  font-family: 'Custom Font';
  src: url(https://mac9416.com/demo/preconnect/font.ttf) format('truetype');
}

body {
  font-family: 'Custom Font';
}

Without resource hints, my page's HTML must be parsed before the stylesheet can be downloaded.
(In this example, I've moved the stylesheet link below a massive block of "lorem ipsum" text, to simulate a late-discovered stylesheet and make the waterfall chart easier to read.)

WebPageTest waterfall chart showing no connection to the external domain until HTML is completely downloaded and parsed

In the above WebPageTest waterfall chart, the page is completely downloaded and parsed before Chrome connects to mac9416.com to download the stylesheet.

preconnect with crossorigin

The asset will load faster if I add a preconnect hint for mac9416.com. (Remember, we're pretending we don't know specifically which assets will be downloaded from mac9416.com. Otherwise we would add preload hints for an even greater performance boost.)

<head>
  <!-- other stuff -->
  <link rel="preconnect" href="https://mac9416.com/" crossorigin>
</head>

WebPageTest waterfall chart showing one connection to the external domain starting immediately after the first chunk of HTML is parsed, but the other one being delayed

With a preconnect hint in place, the waterfall looks better. A DNS lookup for mac9416.com happens immediately after the first chunk of HTML is downloaded. And the connection used to download my custom font happens immediately after the DNS lookup is finished. But it looks like there's another connection to mac9416.com that's initiated after HTML is downloaded and parsed. That second connection is used to download the custom font.

preconnect without crossorigin

In the above code sample, I naiively copied an example and left the crossorigin attribute in place. I just assumed it meant "this connection is to a different domain" -- which it is. Let's see what happens when I remove that attribute.

<head>
  <!-- other stuff -->
  <link rel="preconnect" href="https://mac9416.com/">
</head>

WebPageTest waterfall chart showing one connection to the external domain starting immediately after the first chunk of HTML is parsed, but the other one beind delayed

In this waterfall chart, we seem to have the same problem, but with the connections swapped. The connection used to download styles happens immediately, but the connection used to download fonts begins after the stylesheet is downloaded and parsed.

Why did crossorigin break things?

What's going on? I incorrectly assumed crossorigin simply meant "the target is on another domain." But the browser could infer that by comparing the <link> element's href attribute to the current page's origin. So what is the crossorigin attribute for?

crossorigin actually tells the browser that "resources on this connection are downloaded using CORS."
By default, it specifically means "CORS without credentials."

CORS improves web security. That's all I'll say about it here, because smarter people have explained it better elsewhere.

To speed up this web page, all we need to know is that resources downloaded without CORS are downloaded on a separate connection from those that use CORS.

A quick glance at a list of requests that use CORS shows that our font request will use CORS, but the stylesheet request will not.

preconnect with crossorigin and without

So let's use two preconnect hints, one for non-CORS requests, and the other for CORS requests.

<head>
  <!-- other stuff -->
  <link rel="preconnect" href="https://mac9416.com/">
  <link rel="preconnect" href="https://mac9416.com/" crossorigin>
</head>

WebPageTest waterfall chart showing two connections to mac9416.com starting immediately after the first chunk of HTML is parsed

This waterfall chart looks much better. With two resource hints in place, both connections to mac9416.com are established immediately after the first chunk of HTML is parsed. Incidentally, we've achieved the fastest time to document complete (the blue line in the waterfall chart) of all the tests.

Just for good measure, I'll add a dns-prefetch hint for browsers that don't support the preconnect hint.

<head>
  <!-- other stuff -->
  <link rel="dns-prefetch" href="https://mac9416.com/">
  <link rel="preconnect" href="https://mac9416.com/">
  <link rel="preconnect" href="https://mac9416.com/" crossorigin>
</head>

There's no need for a crossorigin attribute, since DNS queries are performed without CORS.

For this experiment I've ignored crossorigin="use-credentials". I suspect CORS requests with credentials would require a third hint.

Summary

The crossorigin attribute, when used with rel="preconnect", doesn't describe where the target origin is but rather what kind of assets will be downloaded from that origin. If the assets use CORS, crossorigin is needed. If CORS won't be used, crossorigin should be omitted. If both types of assets will be present, two resource hints are necessary.

If you've found resource hints and crossorigin confusing, don't feel bad. There's a lot going on. If you find this guide confusing, please contact me! The fault is probably mine, and I'll be happy to clarify.

Originally posted on crenshaw.dev.

Discussion (3)

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malvoz profile image
Robert Linder

"CORS improves web security. That's all I'll say about it here, because smarter people have explained it better elsewhere."

(while I'm not claiming to be one of the smarter people you're referring to) CORS is actually used to "circumvent" the Same-Origin Policy which is a browser built-in security mechanism that would otherwise not allow for cross-origin resource sharing.

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borisschapira profile image
Boris Schapira

This helped me right now with some waterfalls I couldn't explain. Thanks a lot.

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crenshaw_dev profile image
Michael Crenshaw Author • Edited

You bet! Andy Davies has written a lot about preconnect recently. You might find his work enlightening as well.