GrapesJS

GrapesJS is an open-source, multi-purpose, Web Builder Framework that combines different tools and features with the goal to help you (or users of your application) to build HTML templates without any knowledge of coding. It's a perfect solution to replace the common WYSIWYG editors, which are good for content editing but inappropriate for creating HTML structures. You can see it in action with the official demos, but using its API you're able to build your own editors.