JSFiddle - React, Tailwind, and code Playground

HTML

<div alt="foo">JSLint
    upper
    <div mce_style="test">mce</div>
</div>
    
<div mce_s="test">
    mce
</div>

JavaScript

function getMCE() {
    var el, attr, i, j, arr = [],
        reg = new RegExp('^mce_', 'i'),                //case insensitive mce_ pattern
        els = document.body.getElementsByTagName('*'); //get all tags in body

    for (i = 0; i < els.length; i++) {                 //loop through all tags
        el = els[i]                                    //our current element
        attr = el.attributes;                          //its attributes
        dance: for (j = 0; j < attr.length; j++) {     //loop through all attributes
            if (reg.test(attr[j].name)) {              //if an attribute starts with mce_
                arr.push(el);                          //push to collection
                break dance;                           //break this loop
            }
        }
    }
    return arr;
}

console.log(getMCE())