After update the version to 1.18, I found the required validation is not work on IE 11.

I did some debug and found isContentEditable always true on IE11

<input type="text" id="userName" name="userName" class="form-control" data-val="true" data-val-required="Please input your name">

Debug info

the $element has value

I found 'isContentEditable' is from this commit:
7c22d68