Growing with the Web

Get your nose out of my user agent


Seriously, stop sniffing user agents. It’s hurting web compatibility and causing headaches for both users and browser vendors. Here’s why and what you should be doing instead.

What’s the problem?

To start off, let’s look at what user agents are. The purpose of a user agent is to inform entities external to the browser; what browser is being used, what version it is and the platform it’s running on. To give one of the purest possible examples, here is Netscape 3’s user agent:

Mozilla/3.0 (Win95; I)

Things have changed a lot (for the worse) since then, have a look at Chrome’s current user agent under Windows.

Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.154 Safari/537.36

So why would a string whose purpose is to identify itself as Chrome mention anything about Mozilla or Safari? Because web developers started using this information to deliver the “best experience” for that browser.

Hi, I'm Chrome, Firefox and Safari, and I'll be serving your content today

While there may not have been better alternatives back then, it turned out to be behaviour which was against the web standards of the future. It is now common practice for developers to use user agents to serve customised experiences for particular browsers. The problem is that not everyone uses the major browsers that you test for. What about the person browsing on their Xbox or PS4? What about the person using a lesser known browser? What about web crawlers and accessibility tools? What about browsers in the future that will become popular?

This not only negatively affects users but also causes issues for browser vendors who are essentially forced to follow this behaviour and pretend to be a popular browser, unless they want a subpar experience. In some cases it also hurts the experience on the browsers that you optimise for because browsers are always updating, enabling newer features.

In fact, Mozilla is included in nearly every user agent now which now essentially has no meaning anymore other than “I want to be compatible with websites that work on Netscape and Firefox too”. The same is becoming true for Chrome and Safari in user agents as well.

It’s a vicious cycle that will continue getting worse and worse until web developers consistently use feature detection instead of relying on the user agent.

What’s the solution?

Feature detection

JavaScript feature detection can be used to probe various APIs to check whether features have been implemented properly. This allows enabling/disabling of features based on whether the browser currently supports it, not whether a handful of browsers supported it at the point of development.

function isCanvasSupported() {
  // Check that a created <canvas> element has the getContext function
  return !!document.createElement('canvas').getContext;

if (isCanvasSupported()) {
  // use canvas
} else {
  // use fallback

Modernizr is a feature detection library that makes this process easier, it allows for custom builds so you can save some bytes on features you won’t use.

if (Modernizr.canvas) {
  // use canvas
} else {
  // use fallback

Graceful degredation

Using features that degrade gracefully is another solution to the problem. Where available, these features will just be ignored or skipped if the browser doesn’t support them. An example of this is the transition property in CSS, which is ignored if the browser doesn’t support it, essentially skipping the animation and going to the end state.

.shift {
  padding-left: 0;

  /* ignored if not supported */
  transition: padding-left ease .2s;

.shift:focus {
  padding-left: 1em;

When should user agents be used?

There is one case where you would want to use user agents and that is for analytics. Unfortunately that’s another area that has been hurt by the chain of events. It is a far more complex problem now to figure out what browsers your users are using. Just have a look at the source code for the browser detection library WhichBrowser which attempts to figure out what browser is being used.

Final thoughts

So put in your best effort to do it the right way with feature detection, you will be doing the right thing for both your users and browsers. Who knows, maybe one day browsers will be able to switch them back to what they were designed for.


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