Modal JS by Orblazer

modal.css

.modal-open {
    overflow: hidden;
}

.modal {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-color: rgba(0, 0, 0, 0.5);
    z-index: 10;

    display: flex;
    justify-content: center;
    align-items: center;
    opacity: 0;
    visibility: hidden;

    -webkit-transition: all .3s;
    -moz-transition: all .3s;
    -o-transition: all .3s;
    transition: all .3s;
}

.modal.open {
    opacity: 1;
    visibility: visible;
}

.modal > .modal-container {
    position: relative;
    width: 980px;
    max-height: 90%;
    min-height: 50%;
    background-color: white;
    border-radius: 5px;
    overflow: auto;
    margin-top: -200%;
    opacity: 0;

    -webkit-transition: margin .3s, opacity .3s;
    -moz-transition: margin .3s, opacity .3s;
    -o-transition: margin .3s, opacity .3s;
    transition: margin .3s, opacity .3s;
}

.modal.open > .modal-container {
    margin-top: 0;
    opacity: 1;
}

/**
 * Modal > close
 */
.modal > .modal-container > .close {
    position: absolute;
    color: white;
    font-size: 1.35em;
    top: 10px;
    right: 12px;
    background-color: rgba(0, 0, 0, 0.5);
    padding: 0 8px;
    cursor: pointer;
    z-index: 10;

    -moz-border-radius: 2px;
    -webkit-border-radius: 2px;
    border-radius: 2px;
}

.modal > .modal-container > .close:hover {
    color: black;
}

/**
 * Modal > header
 */
.modal > .modal-container > .modal-header {
    display: flex;
    justify-content: space-between;
}

.modal > .modal-container > .modal-header > .slider {
    width: 49%;
    min-height: 300px;
}

.modal > .modal-container > .modal-header > .header-content {
    width: 49%;
}

.modal > .modal-container > .modal-header > .header-content > .title {
    margin: 0;
    font-size: 2em;
    color: #e8396f;
    padding-top: 15px;
}

.modal > .modal-container > .modal-header > .header-content > .description {
    color: #fab626;
}

.modal > .modal-container > .modal-header > .header-content > p {
    padding-right: 15px;
}

/**
 * Modal > content
 */
.modal > .modal-container > .modal-content {
    min-width: 50px;
    padding: 20px;
}

.modal > .modal-container > .modal-content > h2 {
    margin: 15px 0 0;
    font-size: 1.3em;
    color: #0eb7a3;
}

.modal > .modal-container > .modal-content > h2:first-child {
    margin-top: 0;
}

.modal > .modal-container > .modal-content > p {
    margin: 0;
}

/**
 * Modal > footer
 */
.modal > .modal-container > .modal-footer {
    display: flex;
    justify-content: space-between;
    background-color: #2b2b2b;
    color: white;
    padding: 15px;
}

.modal > .modal-container > .modal-footer > .links {
    margin: 0;
    padding: 0;
    list-style: none;
}

.modal > .modal-container > .modal-footer > .logo > img {
    max-height: 80px;
}

modal.js

var currModal = undefined;
var Modal = function (elem) {
    var self = this;
    var element = elem;
    var closeElem = elem.querySelector('.modal-container>.close');

    if (closeElem instanceof HTMLElement) {
        closeElem.addEventListener('click', function () {
            self.close();
        });
    }

    this.open = function () {
        if (currModal !== undefined) {
            currModal.close();
        }

        if (document.body.className.indexOf('modal-open') === -1) {
            document.body.className += ' modal-open';
        }

        if (element.className.indexOf('open') === -1) {
            element.className += ' open';
        }

        currModal = self;
    };

    this.close = function () {
        if (document.body.className.indexOf('modal-open') !== -1) {
            document.body.className = document.body.className.replace(' modal-open', '');
        }

        if (element.className.indexOf('open') !== -1) {
            element.className = element.className.replace(' open', '');
        }

        currModal = undefined
    }
};