if(eo.scrollTop+eo.clientHeight/4>eos.offsetTop)

.underline {
    border-bottom: 2px solid #DD4B39;
}

function class2re(className)
{
    return new RegExp('(?:^|\\s+)' + className + '(?:\\s+|$)', 'g');
};

function classExists(el, className)
{
    return class2re(className).test(el.className);
};

function addClass(el, className)
{
    if ( classExists(el, className) ) {
        return;
    }
    el.className += ' ' + className;
};

function removeClass(el, className)
{
    var re = class2re(className);
    el.className = el.className.replace(re, ' ');
};

    function test () {
        if (e1.scrollTop > (e2.offsetTop - 20)) {
            addClass(es, 'underline');
            removeClass(ef, 'underline');
        } else {
            removeClass(es, 'underline');
            addClass(ef, 'underline');
        }