How many different smartphones and tablets can you test on?
The big hitters - iOS (Apple) and Android (Google) sounds like two but it is not! Just with Apple there is iPhone 3GS, iPhone 4, iPhone 4S, iPad, iPad2 and iPad3. Then there are various levels of iOS version which include tweaks to Safari. Just testing on each main Apple device and two OS levels say 4.2 and 5.1 is 12 different targets to test on.
Then adding Android makes things really complicated, there are dozens of different devices all with different screen ratios and different resolutions. Android also offers different browsers, such a the popular Opera Mini. These different combination would deliver 100s of different targets to test on.
Clearly, comprehensive cross platform testing is not feasible, not only would it cost tens of thousands of pounds in devices but it would take thousands of hours. The professional mSite development shops have an array of most popular devices and based on experience test on the key platforms. Even if this is just 8 devices it still takes hours, controlling each device is slow, then tweaking CSS and retesting on the device increases the time it takes. Trust me, one simple mSite page can take hours to test and tweak cross major platforms.
Adobe have observed this pain point in mSite development. From inside the Adobe Labs there is a new product currently in beta called Shadow. Adobe Shadow is free to download.
So what does it do? It enables you to load an mSite on many devices at the same time from your computer. As you browse on the computer all the devices follow. Just this is a serious time saver, but they empower the developer to target one of the devices and change the CSS in realtime to fix a bug. This brings the power of the browser "developer toolbar" to mSite development.
You can watch a video from adobe here that shows off Shadow.