How do I add "show html" button in Quill like in Wysiwyg?

Well this was still an issue 3 years later.

So I made a little module to accomplish this.

https://github.com/benwinding/quill-html-edit-button

If you check the following link, you will see that they don't add the option for an HTML button to the configuration list keys. Probably, you have to extend the library. http://quilljs.com/docs/configuration/ is the link to the configuration page.

In the API calls list I saw that you can do a function call to get HTML from the plugin. You can find it in the following link. The function name is called getHTML(). So you might have to extend the library by creating your own button and a display area.