This page provides an overview of some of the main techniques needed to design web sites that work well on mobile devices. If you're looking for information on Mozilla's Boot to Gecko project, see the Boot to Gecko page. Or you might be interested in details about Firefox for Android.
We've organized it into two sections, designing for mobile devices and cross-browser compatibility. Also see Jason Grlicky's guide to mobile-friendliness for web developers.
Designing for mobile devices
Mobile devices have quite different hardware characteristics compared with desktop or laptop computers. Their screens are usually smaller, obviously, but they also usually automatically switch the screen orientation between portrait and landscape mode as the user rotates the device. They usually have touch screens for user input. APIs like geolocation or orientation are either not supported on desktops or are much less useful, and these APIs give mobile users new ways to interact with your site.
Working with small screens
Responsive Web Design is a term for a set of techniques that enables your web site to adapt its layout as its viewing environment — most obviously, the size and orientation of the screen — changes. It includes techniques such as:
- fluid CSS layouts, to make the page adapt smoothly as the browser window size changes
- the use of media queries to conditionally include CSS rules appropriate for the device screen width and height
The viewport meta tag instructs the browser to display your site at the appropriate scale for the user's device.
Working with touch screens
To use a touch screen you'll need to work with DOM Touch events. You won't be able to use the CSS :hover pseudo-class, and will need to design clickable items like buttons to respect the fact that fingers are fatter than mouse pointers. See this article on designing for touch screens.
You can use the -moz-touch-enabled media query to load different CSS on a touch-enabled device.
Optimizing images
To help users whose devices have low or expensive bandwidth, you can optimize images by loading images appropriate to the device screen size and resolution. You do this in CSS by querying for screen height, width, and pixel ratio.
You can also make use of CSS properties to implement visual effects like gradients and shadows without images.
Mobile APIs
Finally, you can take advantage of the new possibilities offered by mobile devices, such as orientation and geolocation.
Cross-browser development
Write cross-browser code
To create web sites that will work acceptably across different mobile browsers:
- Try to avoid using browser-specific features, such as vendor-prefixed CSS properties.
- If you do need to use these features, check whether other browsers implement their own versions of these features, and target them too.
- For browsers that don't support these features, provide an acceptable fallback.
For example, if you set a gradient as a background for some text using a vendor-prefixed property like -webkit-linear-gradient, it's best to include the other vendor-prefixed versions of the linear-gradient property. If you don't do that, at least make sure that the default background contrasts with the text: that way, the page will at least be usable in a browser which is not targeted by your linear-gradient rule.
See this list of Gecko-specific properties, and this list of WebKit-specific properties, and Peter Beverloo's table of vendor-specific properties.
Using tools like CSS Lint can help find problems like this in code, and preprocessors like SASS and LESS can help you to produce cross-browser code.
Take care with user agent sniffing
It's preferable for web sites to detect specific device features such as screen size and touch screens using the techniques listed above, and adapt themselves accordingly. But sometimes this is impractical, and web sites resort to parsing the browser's user agent string to try to distinguish between desktops, tablets, and phones, to serve different content to each type of device.
If you do this, make sure your algorithm is correct, and you aren't serving the wrong type of content to a device because you don't understand a partcular browser's user agent string. See this guide to using the user agent string to determine device type.
Test on multiple browsers
Test your web site on multiple browsers. This means testing on multiple platforms — at least iOS and Android.
- test mobile Safari on the iPhone using the iOS simulator
- test Opera and Firefox using the Android SDK. See these additional instructions for running Firefox for Android using the Android emulator.