Restrict to designing purely with CSS styles/properties, match browser's rendering appearance
As has been noted for this to be a good web app it will need to fully embrace css and render it accurately.
Therefore it would be useful to tag objects or artboards to display in strict mode - exactly according to css properties - or to display in lets say design mode to allow freeform placement of objects which would require additional markup in development to work properly.
To be a killer web design tool this app needs to have a full css toolset at least as far attributes that are key to design and presentation and artboards that render accurately according to css properties.
This should include definable breakpoints that are detected by artboards and the ability link objects across artboards under global styles as has been noted elsewhere to show responsive variation.
The ability to export css, not necessarily as a complete production ready stylesheet but in a form that i can extract assets from for production instead of recreating everything.
The functionality of this program should match what's possible in HTML 5 EXACTLY.