I don't know... few classes in the CSS but tons of classes in the attributes vs tons of classes in the CSS but few classes in the attributes. I somewhat doubt it minimizes layout calculation much, many styles are still being applied to many DOM nodes. Memory for those classes, maybe, but I doubt lookup costs very much compared to other parts of rendering a page.

Though I would be interested in seeing some profiling data demonstrating one way or the other. Even if it's relatively transient data, as it relies heavily on the rendering code.