TinyMCE 4.0 requires text/css for editor style files

As of TinyMCE 4.0, the visual editor iframe now has an HTML5 document type (<!DOCTYPE html>). In this scenario, CSS files must be served with the text/css content type. A server will serve a *.css file with the proper content type, but if you’re using a PHP file for an editor style file, you need to be the one to do it. It’s as simple as leading with:

<?php
header( 'Content-Type: text/css; charset=UTF-8' );

So if you’re doing something particularly crazy with the editor and your styles aren’t loading in WordPress 3.9, you may just need a content type. Also, Chrome (and probably other browsers) throw a console warning when this happens.

(via #27288)