TinyMCE 4.0 requires text/css for editor style files

As of TinyMCE 4.0, the visual editor iframeiframe iFrame is an acronym for an inline frame. An iFrame is used inside a webpage to load another HTML document and render it. This HTML document may also contain JavaScript and/or CSS which is loaded at the time when iframe tag is parsed by the user’s browser. now has an HTML5 document type (<!DOCTYPE html>). In this scenario, CSSCSS Cascading Style Sheets. 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 PHPPHP The web scripting language in which WordPress is primarily architected. WordPress requires PHP 5.6.20 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)

#3-9, #dev-notes, #editor, #tinymce