Cannot create element with a class, reserved keyword.

A fixed version of your function, using cls to set the 'class' attribute.

var Element = function(type, atts, content) {

this.el = document.createElement(type);

for(var attr in atts) {
    if (attr == 'cls') {
        this.el.setAttribute('class', atts[attr]);
    } else {
        this.el.setAttribute(attr, atts[attr]);
    }
}

this.el.innerHTML = content || '';
return this.el;

};