• Copyright 2011: Scott Jehl, scottjehl.com
• Dual licensed under the MIT or GPL Version 2 licenses.

The goal of this script is to provide a fast and lightweight (3kb minified / 1kb gzipped) script to enable responsive web designs in browsers that don't support CSS3 Media Queries - in particular, Internet Explorer 8 and under. It's written in such a way that it will probably patch support for other non-supporting browsers as well (more information on that soon).

If you're unfamiliar with the concepts surrounding Responsive Web Design, you can read up here and also here

Demo page (the colors change to show media queries working)

    @media screen and (min-width: 480px){
        ...styles for 480px and up go here
    }

Some notes to keep in mind:

• This script's focus is purposely very narrow: only min-width and max-width media queries and all media types (screen, print, etc) are translated to non-supporting browsers. I wanted to keep things simple for filesize, maintenance, and performance, so I've intentionally limited support to queries that are essential to building a mobile-first responsive design. In the future, I may rework things a bit to include a hook for patching-in additional media query features - stay tuned!

• Browsers that natively support CSS3 Media Queries are opted-out of running this script as quickly as possible. In testing for support, I immediately pass browsers that support the window.matchMedia API (such as recent Chrome releases), and Internet Explorer 9+. All other browsers are subjected to a quick feature test to determine whether they support media queries or not before proceeding to run the script.

• This script relies on no other scripts or frameworks, and is optimized for mobile delivery (~1kb total filesize)

• As you might guess, this implementation is quite dumb in regards to CSS parsing rules. This is a good thing, because that allows it to run really fast, but it's looseness may also cause unexpected behavior. For example: if you enclose a whole media query in a comment intending to disable its rules, you'll probably find that those rules will end up enabled in non-media-query-supporting browsers.

• Respond.js doesn't parse CSS refrenced via @import, nor does it work with media queries within style elements, as those styles can't be re-requested for parsing.

• Due to security restrictions, some browsers may not allow this script to work on file:// urls (because it uses xmlHttpRequest). Run it on a web server.

• Due to its use of Ajax, this script will only work with same-domain CSS files, though I may patch that up soon.

• Currently, media attributes on link elements are supported, but only if the linked stylesheet contains no media queries. If it does contain queries, the media attribute will be ignored and the internal queries will be parsed normally.

• WARNING: Do not include @font-face rules inside a media query. This will crash IE7 and IE8. Simply place @font-face rules in the wide open, as a sibling to other media queries. Isolated test here to demostrate (note: test crashes IE 7&8): http://jsfiddle.net/scottjehl/Ejyj5/1/

Basically, the script loops through the CSS referenced in the page and runs a regular expression or two on their contents to find media queries and their associated blocks of CSS. Since at least in Internet Explorer, the content of the stylesheet is impossible to retrieve in its pre-parsed state (which in IE 8-, means its media queries are removed from the text), Respond.js re-requests the CSS files using Ajax and parses the text response from there. Be sure to configure your CSS files' caching properly so that this re-request doesn't actually go to the server.

From there, each query block is appended to the head in order via style elements, and those style elements are enabled and disabled (read: appended and removed from the DOM) depending on how their min/max width compares with the browser width. The media attribute on the style elements will match that of the query in the CSS, so it could be "screen", "projector", or whatever you want. Any relative paths contained in the CSS will be prefixed by their stylesheet's href, so image paths will direct to their proper destination

Sure, a couple:

• respond.update() : rerun the parser (helpful if you added a stylesheet to the page and it needs to be translated)
• respond.mediaQueriesSupported: set to true if the browser natively supports media queries

This isn't the only CSS3 Media Query polyfill script out there; but it damn well may be the fastest.

If you're looking for more robust CSS3 Media Query support, you might check out http://code.google.com/p/css3-mediaqueries-js/. In testing, I've found that script to be noticeably slow when rendering complex responsive designs (both in filesize and performance), but it really does support a lot more media query features than this script. Big hat tip to the authors! :)