Live Preview / WYSIWYG support (plugin?) for Notepad++?

The plugin Preview HTML is the closest match to what your are looking for.

Go to the Plugins menu and then find the Preview HTML entry to install it. Its description is: Preview HTML files inside Notepad++ (or in a floating window) without having to save them first. The latest update now refreshes the preview automatically after switching tabs, or whenever the document is changed.

More information in the official website.