An important clarification from Stephen:
You don’t actually design in the browser
When I speak of designing in the browser, I mean creating browser-based design mockups/comps (I use the terms interchangeably), as opposed to static comps (like the PSDs we’re all used to). So it’s not the design. It’s the visualization of the design—the one you present to stakeholders.
Personally, I think it’s as crazy to start in the browser as it is to start with Photoshop—both have worldviews and constraints that will affect your thinking. Start with paper.