I run www.mobjectify.com

While it is targeted primarily towards users developing mobile sites or apps, it does solve atleast two of the problems you stated.

The mockups are made using jQuery Mobile and so are responsive by nature, and the design can easily be exported into html and css.

While it is still far from perfect, it does work with the aim of the browser being your mockup tool, with the service simply being an aid to allow access to prebuilt components.