Freezing User-Agent Strings

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Chris Coyier on

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There’s been news about Chrome freezing their User-Agent string (and all other major browsers are on board). That means they’ll still have a User-Agent (UA) string (that comes across in headers and is available in JavaScript as navigator.userAgent. By freezing it, it will be less useful over time in detecting the browser/platform/version, although the quoted reason for doing it is more about privacy and stopping fingerprinting rather than developer concerns.

In the front-end world, the general advice is: you shouldn’t be doing UA sniffing. The main problem is that so many sites get it wrong, and the changes they make with that information ends up hurting more than it helps. And the general advice for avoiding it is: you should test based on the reality of what you are trying to do instead.

Are you trying to test if a browser supports a particular feature? Then test for that feature, rather than the abstracted idea of a particular browser that is supposed to support that feature.

In JavaScript, sometimes features are very easy to test because you test for the presence of their APIs:

  if (navigator.geolocation) {
  } else {
    console.warn("Geolocation not supported");

In CSS, we have a native mechanism via @supports:

@supports (display: grid) {
  .main {
    display: grid;

That is exposed in JavaScript via an API that returns a boolean answer:

CSS.supports("display: flex");

Not everything on the web platform is this easy to test, but it’s generally possible without doing UA sniffing. If you’re in a difficult position, it’s always worth checking to see if Modernizr has a test for it, which is kinda the gold-standard of feature testing as chances are it has been battle-tested and has dealt with edge cases in a way you might not foresee. If you actually use the library, it gives you clean logical breaks:

if (Modernizr.requestanimationframe) {
  // supported
} else {
  // not-supported

What if you just really need to know the browser type, platform, and version? Well, apparently that information is still possible to get, via a new thing called User-Agent Client Hints (UA-CH).

Wanna know the platform? You set a header on the request called Sec-CH-Platform and theoretically, you’ll get that information back in the response. You have to essentially ask for it, which is apparently enough to prevent the problematic privacy fingerprinting stuff. It appears there are headers like Sec-CH-Mobile for mobile too, which is a little curious. Who is deciding what a “mobile” device is? What decisions are we expected to make with that?

Knowing information about the browser, platform and version at the server level if often desirable as well (sending different code in different situations) — just as much as it is client-side, but without the benefit of being able to do tests. Presumably, the frozen UA strings will be useful for long enough that server-side situations can port over to using UA-CH.

Jon Arne Sæterås is nervous:

Professionally, I’ve been hands on with the mobile web space and seen it develop for more than 15 years and I know that many, big and small, websites rely on device detection based on the User-Agent header. From Google’s perspective it may seem easy to switch to the alternative UA-CH, but this is where the team pushing this change doesn’t understand the impact:

Functionality based on device detection is critical, widespread and not only in front end code. Huge software systems with backend code rely on device detection, as well as entire infrastructure stacks.

In my most major codebase, we do a smidge of server-side UA detection. We use a Rails gem called Browser that exposes UA-derived info in a nice API. I can write:

if browser.safari?


We also expose information from that gem on the client-side so it can be used there as well. There is only a handful of instances of usage for both front and back, none of which look like they would be particularly difficult to handle in some other way.

In the past it’s been kinda tricky to relay front-end information back to the server in such a way that’s useful on the first page load (since the UA doesn’t know stuff like viewport size). I remember some pretty fancy dancing I’ve done where I load up a skeleton page that executes a tiny bit of JavaScript that did things like measure the viewport width and screen size, then set a cookie and force-refreshed the page. If the cookie was present, the server had what it needed and didn’t load the skeleton page at all on those requests.

Tricky stuff, but then the server has information about the viewport width on the server-side, which is useful for things, like sending small-screen assets (e.g.different HTML), which was otherwise impossible.

I mention that because UA-CH stuff is not to be confused with regular ol’ Client Hints. We’re supposed to be able to configure our servers to send an Accept-CH header and then have our client-side code whitelist stuff to send back, like:

<meta http-equiv="Accept-CH" content="DPR, Viewport-Width">

That means a server can have information from the client about these things on subsequent page loads. That’s a nice API, but Firefox and Safari don’t support it. I wonder if it will get a bump if both of those browsers are signaling interest in UA-CH because of this frozen UA string stuff.