<div class="div">div</div>
<a href="#" class="close"> Закрыть </a>

var closeBtn = document.getElementsByClassName('close')[0];

closeBtn.addEventListener('click', function() {
    document.getElementsByClassName('div')[0].style.display = 'none';
});

@media (max-width: 767px) {
        /*xs*/
        .btn {
            padding: 1px 5px;
            font-size: 12px;
            line-height: 1.5;
            border-radius: 3px;
        }
    }
    @media (min-width: 768px) and (max-width: 991px) {
        /*sm*/
        .btn {
            padding: 5px 10px;
            font-size: 12px;
            line-height: 1.5;
            border-radius: 3px;
        }
    }
    @media (min-width: 992px) and (max-width: 1199px) {
        /*md*/
        .btn {
            padding: 6px 12px;
            font-size: 14px;
            line-height: 1.42857143;
            border-radius: 4px;
        }
    }
    @media (min-width: 1200px) {
        /*lg*/
        .btn {
            padding: 10px 16px;
            font-size: 18px;
            line-height: 1.3333333;
            border-radius: 6px;
        }
    }

var html = document.createElement('div');

html.innerHTML = '<div id="content"></div>';

console.dir(html.children[0]);