Luke Wroblewski has summarized some of the RWD and mobile design challenges, and created a catalog of multi-device layout patterns. He suggests that, compared with a simple RWD approach, device experience or RESS (responsive web design with server-side components) approaches can provide a user experience that is better optimized for mobile devices. Server-side "dynamic CSS" implementation of stylesheet languages like Sass or Incentivated's MML can be part of such an approach by accessing a server based API which handles the device (typically mobile handset) differences in conjunction with a device capabilities database in order to improve usability. RESS is more expensive to develop, requiring more than just client-side logic, and so tends to be reserved for organisations with larger budgets.
Although this challenge has become recently a minor issue, with many of the publishers starting to support responsiveness, one still at least partly existing problem for RWD is that some banner advertisements and videos are not fluid. However search advertising and (banner) display advertising support specific device platform targeting and different advertisement size formats for desktop, smartphone, and basic mobile devices. Different landing page URLs can be used for different platforms, or Ajax can be used to display different advertisement variants on a page.
An alternative to RWD is the method of Adaptive Web Delivery or AWD that is adopted by consumer brands worldwide. Although, it is very similar to Responsive Web Design, with adaptive delivery, the most significant difference is that the server hosting the website detects the devices making requests to it, and uses this information to deliver different batches of HTML and CSS code based on the characteristics of the device that have been detected.
There are now many ways of validating and testing RWD designs, ranging from mobile site validators and mobile emulators to simultaneous testing tools like Adobe Edge Inspect. The Firefox browser and the Chrome console offer responsive design viewport resizing tools, as do third parties.