WordPress defaults to a WYSIWYG editor when composing a new Post. Of course WYSIWYG is a bit of a misnomer. What you "get" when you publish that post is dependent on the template and the CSS in place in the theme. In fact, WordPress doesn't even call it WYSIWYG, they call it the "Visual" editor. In fact, most editors of this nature these days go to length in telling you its a markup editor, not a WYSIWYG editor.