FYI to you all,
This is also a tinyMCE bug (that should ignore this setting in IE/Edge to prevent the exception)
In IE/Edge run the following at the console:
document.querySelector("body").dir="rtl"; //"rtl"
document.querySelector("body").dir="ltr"; //"ltr"
document.querySelector("body").dir="auto"; //Invalid argument.
you get this error when passing the setting directionality="auto" in tinyMCE (it should ignore it..)
The error you see at the console is Invalid argument at N.prototype.initContentBody ... at the tinyMCE source