#footer {
  position: absolute;
  bottom: 1ex;
  margin: 1ex 1em;
  padding-top: 0.5ex;
  border-top: thin solid #999;
  font-size: 80%;
  width: 95%;
}
#footer .copyright,
#footer .reference {
    display: inline;
    width: 50%;
    float: left;
    text-align: left;
}
#footer .reference {
    float: right;
    text-align: right;
}

/* ref: http://codepen.io/ShadyAlzayat/pen/LEDJg */
/* [Object] Modal
 * =============================== */
.modal {
  opacity: 0;
  visibility: hidden;
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  text-align: left;
  background: rgba(0,0,0, .7);
  transition: opacity .25s ease;
}

.modal__bg {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  cursor: pointer;
}

.modal-state {
  display: none;
}

.modal-state:checked + .modal {
  opacity: 1;
  visibility: visible;
}

.modal-state:checked + .modal .modal__inner {
  top: 0;
}

.modal__inner {
  transition: top .25s ease;
  position: absolute;
  top: -20%;
  right: 0;
  bottom: 0;
  left: 0;
  width: 50%;
  margin: auto;
  overflow: auto;
  background: #fff;
  border-radius: 5px;
  padding: 1em 2em;
  height: 50%;
}

.modal__close {
  position: absolute;
  right: 1em;
  top: 1em;
  width: 1.1em;
  height: 1.1em;
  cursor: pointer;
}

.modal__close:after,
.modal__close:before {
  content: '';
  position: absolute;
  width: 2px;
  height: 1.5em;
  background: #ccc;
  display: block;
  transform: rotate(45deg);
  left: 50%;
  margin: -3px 0 0 -1px;
  top: 0;
}

.modal__close:hover:after,
.modal__close:hover:before {
  background: #aaa;
}

.modal__close:before {
  transform: rotate(-45deg);
}

@media screen and (max-width: 768px) {
    
  .modal__inner {
    width: 90%;
    height: 90%;
    box-sizing: border-box;
  }
}

.btn {
  cursor: pointer;
  text-decoration: underline;
  color: -webkit-link;
}