Basically, you want to build an open source (probably browser-based) tool that allows you to drag and drop different elements to build out an HTML5 web page/s?

Basically, you want to build an open source (probably browser-based) tool that allows you to drag and drop different elements to build out an HTML5 web page/s? Quite a bit of work, but could be done for design purposes (I'd suggest avoiding any JavaScript functionality for now).