Modernizr.plaintextOnly test for <div contentEditable="plaintext-only">. Currently only works in webkit. see http://stackoverflow.com/questions/10672081/how-to-detect-if-browsr-supports-plaintext-only-value-in-contenteditable-para for more details.

modernizr.tests.plaintextonly.js

//source
//http://stackoverflow.com/questions/10672081/how-to-detect-if-browser-supports-plaintext-only-value-in-contenteditable-para
Modernizr.addTest('plaintextonly', function(){
    var d = document.createElement("div");
    try {
        d.contentEditable="PLAINtext-onLY";
    } catch(e) {
        return false;
    }
    return d.contentEditable=="plaintext-only";
});