Congratulations! Glad to see more people attempting this type of thing. The demo content design is pretty nice.
Some good things in Easel, but I have lots of problems with all the tiny little controls. Also it's an interesting decision to use a canvas element instead of the DOM. Without solving the overall page layout issue, this is a nice drawing tool like others have mentioned, and kudos to the technical and design effort, but in the end if you're not using the DOM, you're making pictures of web sites...
Therefore, it's my responsibility to let you all know about Edit Room, my project as an independent developer, http://www.edit-room.com/
Edit Room is also a 'web design in the browser' app, but it uses the real DOM, not a canvas, so whatever you can build with Edit Room is what is truly able to be built with HTML and CSS. It's only a slight limitation due to the unique way I've handled absolute/relative layouts. Edit Room still gives you incredible design and layout freedom, without floats.
Edit Room doesn't do fixed widths at all. Documents created with Edit Room flex naturally. Everything you layout and design with my app is based on a flexible grid, and it has a built-in width preview, so you can design at any screen width.
Edit Room exports the full HTML and CSS for your entire layout, or you can copy individual elements' CSS, it's adaptable to your workflow.
Edit Room also supports linking Typekit and Fontdeck accounts, so you can use Web Fonts in your exported designs.
You can try out the demo here: http://www.edit-room.com/screens/13/edit
Oh, and one more thing: You can animate your layers with visual keyframes and Edit Room generates CSS3 Animations.