Feature detection with Modernizr and Polyfills

Not all features of the web are equal. The web is young and in flux. Things change, things break. When we build things we rely on different browser features to achieve our goals. Perhaps we need to use css-transitions or touch events.

In the spirit of progressive enhancement, graceful degradation or progressive reduction we have learned that it is a good idea to test what the user’s platform supports before we start using features that would break anyway.

You need to know, your code needs to know

We need to know what features are supported where. Our code needs to know. And we need to know what to do with when a feature isn’t supported, for instance polyfilling.

When Can I Use tells you the browser support story, while Modernizr gives you the power of feature detection. HTML5 Please helps you out with recommendations for polyfills and implementation so you can decide if and how to put each of these features to use. (quote from HTML5 please)

Avoid browser sniffing

Before Modernizr and such precise tools arrived, browser sniffing was used all the time. The concept is to test for a user-agent header which the browser sends, revealing which browser it is. So you knew that if it was Internet Explorer 8, this and that feature isn’t supported.

This very quickly turned into a mess. The problem with this method is that you don’t write your code based to the feature but to a specific browser version. Browsers get updated while versions numbers and features fly around. It is not a very reliable method.

Instead, you could and probably should use feature detection!

Feature detection

Modernizr.js is a JavaScript library “that detects HTML5 and CSS3 features in the user’s browser”.

Taking advantage of cool new web technologies is great fun, until you have to support browsers that lag behind. Modernizr makes it easy for you to write conditional JavaScript and CSS to handle each situation, whether a browser supports a feature or not. It’s perfect for doing progressive enhancement easily.


Task: Rotate an element 90 degrees First solution: css div { transform: rotate(90); }

This will do the job, but what if the browser does not support CSS transforms? In most cases, nothing happens as the browser happily ignores the rule but now we when the element isn’t rotated the design doesn’t work anymore. We need a fallback. Enter Modernizr.

We need Modernizr to feature detect for CSS transforms so we can progressively enhance our element. This means that we will first write the code for a design without transforms and enhance with code for a design with transforms using the class modernizr automatically adds to the


It will look something like this:

<html lang="en" dir="ltr" class="js no-touch postmessage history multiplebgs boxshadow opacity cssanimations csscolumns cssgradients csstransforms csstransitions fontface localstorage sessionstorage svg inlinesvg no-blobbuilder blob bloburls download formdata">

All these classes indicated what is currently supported in the browser loading the page.

In our case, the browser does support CSS transforms as indicated by the csstransforms class.

New solution

div {
	/* code that makes it look good without the rotate */
.csstransforms div {
	transform: rotate(90);
	/* code that makes it look even better now that we support rotate */

This way you will have used feature detection to make sure your design looks good no matter what is supported.

How it really works

Modernizr runs quickly on page load to detect features; it then creates a JavaScript object with the results, and adds classes to the html element for you to key your CSS on. Modernizr supports dozens of tests…

Customizing Modernizr

You can use Modernizr out of the box and feature detect for every possible feature but you the smart developer would never do that. You are going to build a customized version of Modernizr.js that only tests the features you are actually using.

Less tests, less work, less bytes, better performance.

You can build your own production-ready Modernizr at modernizr.com/download.


The battle of support and optimization