Unfortunately, I don't think I can fix this. This is the default behavior of TinyMCE. To override it, we have to give TinyMCE an explicit width (like 100%), but that is unknown. To get that, we actually need the percentage width in the CSS, but JS will basically only give us the final computed value (see http://ie.microsoft.com/testdrive/HTML5/getComputedStyle/Default.html ).