I'm waiting for a feature to drag borders between divs to play with padding and margins. And how about pinch to zoom font sizes. Then when I've got things the way I like them I want a CSS diff view to change the actual files. Better yet, I'd like to tell the browser the local path to my CSS files and have it update them for me. Now that would be a development browser.
Just gave it a look and it has some nice features. The 3D view offers an interesting perspective, but I'm not sure if I'd use that for anything. I think the most useful feature is the responsive view. Apple should add that to Safari. The main thing that bugged me was the dark theme, but I figured out how to turn that off, so I might just try this for a project or two.