Step 1 was to supply some sample code. Step 2 was to fill in an online Google docs form asking some quite detailed and technical questions. The guideline time for the form was 1 hour, but it took me quite a bit longer more like 10 hours. In the form they had some questions on high level architecture, asked questions about their application and how it worked and how I would implement it, but the main one was to implement flexible graph plotting code to output 3 different types of chart on an HTML canvas. Step 3 was a telephone interview during which they asked questions about OO programming and its implementation in JS, basic CSS and a few JS questions. Step 4 was a face to face interview. During the interview they had a brief set of reasonably basic JS and CSS questions and then moved on to discuss the code I'd written in step 1 and step 3.
At the end of the interview I was given a response straight away (no in my case) and a very decent opportunity to ask for feedback which they provided in detail.
They were quite inflexible in their style of interview, in that they obviously wanted very specific answers, but I think that reflects that they were looking for a very specific skill set. There was no willingness to allow for that skill set to be acquired, they wanted someone who had this skill set yesterday.
Overall they wanted to see a strong inclination to write your own JavaScript code without relying on any libraries (I would recommend not mentioning or using a single one during the interview, because they took this as a sign of weakness, even though I'm very strongly inclined to write my own code) and the ability to demonstrate a very strong understanding of design patterns (in particular, I would recommend any front end developer hoping to be successful here to have a very good understanding of the design principles behind the drawing API in Java's AWT or/and Swing).
A final point that I would make is that they really want you to have experience with graphics rendering even though when I was offered an interview this was on the list of advantageous skills. I feel that they could have worked this out at a much earlier stage - I certainly didn't put it on my CV!