Best CSS gradient, shadow, radius, transition and transform WYSIWYG editor I’ve seen to date. You can create instances for :hover, :active, custom class states. You can also work with transitions and transforms.

FROONT seems to be worth playing with.

jQuery, 5KB.

Jetstrap may be useful.

Via @jakubspanihel

wysihtml5 is an open source rich text editor based on HTML5 technology and the progressive-enhancement approach. It uses a sophisticated security concept and aims to generate fully valid HTML5 markup by preventing unmaintainable tag soups and inline styles.

Via @ViliamKopecky