(For more resources on this subject, see here.)

Detect and design with features, not User agents (browsers)

What if you could build your website based on features instead of for the individual browser idiosyncrasies by manufacturer and version, making your website not just backward compatible but also forward compatible? You could quite potentially build a fully backward and forward compatible experience using a single code base across the entire UA spectrum. What I mean by this is instead of baking in an MSIE 7.0 version, an MSIE 8.0 version , a Firefox version, and so on of your website, and then using JavaScript to listen for, or sniff out, the browser version in play, it would be much simpler to instead build a single version of your website that supports all of the older generation, latest generation, and in many cases even future generation technologies, such as a video API,box-shadow,and first-of-type.

Think of your website as a full-fledged cable television network broadcasting over 130 channels, and your users as customers that sign up for only the most basic package available, of only 15 channels. Any time that they upgrade their cable (browser) package to one offering additional channels (features), they can begin enjoying them immediately because you have already been broadcasting to each one of those channels the entire time.

What happens now is that a proverbial line is drawn in the sand, and the site is built on the assumption that a particular set of features will exist and are thus supported. If not, fallbacks are in place to allow a smooth degradation of the experience as usual, but more importantly the site is built to adopt features that the browser will eventually have. Modernizr can detect CSS features, such as @font-face , box-shadow , and CSS gradients. It can also detect HTML5 elements, such as canvas , localstorage, and application cache. In all it can detect over 40 features for you, the developer.
Another term commonly used to describe this technique is "progressive enhancement ". When the time fi nally comes that the user decides to upgrade their browser, the new features that the more recent browser version brings with it, for example text-shadow , will automatically be detected and picked up by your website, to be leveraged by your site with no extra work or code from you when they do. Without any additional work on your part, any text that is assigned text-shadow attributes will turn on at the fl ick of a switch so that user's experience will smoothly, and progressively be enhanced.

What is Modernizr? More importantly, why should you use it? At its foundation, Modernizr is a feature-detection library powered by none other than JavaScript.

Here is an example of conditionally adding CSS classes based on the browser, also known as the User Agent . When the browser parses this HTML document and fi nds a match, that class will be conditionally added to the page.

Now that the browser version has been found, the developer can use CSS to alter the page based on the version of the browser that is used to parse the page. In the following example, IE 7, IE 8, and IE 9 all use a different method for a drop shadow attribute on an anchor element:

/* IE7 Conditional class using UA sniffing */
.lt-ie7 a{ display: block; float: left; background: url
( drop-shadow.
gif ); }
.lt-ie8 a{ display: inline-block; background: url
( drop-shadow.png );  
}  
.lt-ie9 a{ display: inline-block;  box-shadow: 10px 5px 5px 
rgba(0,0,0,0.5); }

The problem with the conditional method of applying styles is that not only does it require more code, but it also leaves a burden on the developer to know what browser version is capable of a given feature, in this case box-shadow . Here is the same example using Modernizr . Note how Modernizr has done the heavy lifting for you, irrespective of whether or not the box-shadow feature is supported by the browser:

/* Box shadow using Modernizr CSS feature detected classes */
.box-shadow a{ box-shadow: 10px 5px 5px rgba(0,0,0,0.5); }
.no-box-shadow a{ background: url( drop-shadow.gif ); }

The Modernizr namespace

The Modernizr JavaScript library in your web page's header is a lightweight feature library that will test your browser for the features that it may or may not support, and store those results in a JavaScript namespace aptly named Modernizr. You can then use those test results as you build your website.
From this point on everything you need to know about what features your user's browser can and cannot support can be checked for in two separate places. Going back to the cable television analogy, you now know what channels (features) your user does and does not have, and can act accordingly.