a.css

/* Pure CSS3 Lightbox start */
a.button {
  display: inline-block;
  background: #999;
  color: #f2f2f2;
  -moz-border-radius: 5px;
  -webkit-border-radius: 5px;
  border-radius: 5px;
  padding: 10px 1em;
  -moz-box-shadow: 0 3px 0 #777;
  -webkit-box-shadow: 0 3px 0 #777;
  box-shadow: 0 3px 0 #777;
  font-size: 1.3em;
  text-decoration: none;
}

.lightbox-target {
  position: fixed;
  top: -100%;
  width: 100%;
  background: rgba(0, 0, 0, 0.7);
  opacity: 0;
  -moz-transition: opacity 0.5s ease-in-out;
  -o-transition: opacity 0.5s ease-in-out;
  -webkit-transition: opacity 0.5s ease-in-out;
  transition: opacity 0.5s ease-in-out;
  overflow: hidden;
}
.lightbox-target .content {
  width: 90%;
  height: 90%;
  background: #fff;
  color: #333;
  margin: auto;
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  max-height: 0%;
  max-width: 0%;
  border: 3px solid #fff;
  -moz-box-shadow: 0 0 8px rgba(0, 0, 0, 0.3);
  -webkit-box-shadow: 0 0 8px rgba(0, 0, 0, 0.3);
  box-shadow: 0 0 8px rgba(0, 0, 0, 0.3);
  box-sizing: border-box;
  -moz-transition: ease-in-out 0.5s;
  -o-transition: ease-in-out 0.5s;
  -webkit-transition: ease-in-out 0.5s;
  transition: ease-in-out 0.5s;
}
.lightbox-target img {
  margin: auto;
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  max-height: 0%;
  max-width: 0%;
  border: 3px solid #fff;
  -moz-box-shadow: 0 0 8px rgba(0, 0, 0, 0.3);
  -webkit-box-shadow: 0 0 8px rgba(0, 0, 0, 0.3);
  box-shadow: 0 0 8px rgba(0, 0, 0, 0.3);
  box-sizing: border-box;
  -moz-transition: ease-in-out 0.5s;
  -o-transition: ease-in-out 0.5s;
  -webkit-transition: ease-in-out 0.5s;
  transition: ease-in-out 0.5s;
}
.lightbox-target:target {
  opacity: 1;
  top: 0;
  bottom: 0;
}
.lightbox-target:target .content, .lightbox-target:target img {
  max-height: 100%;
  max-width: 100%;
}
.lightbox-target:target .lightbox-close {
  top: 5%;
}

.lightbox-close {
  display: block;
  width: 50px;
  height: 50px;
  box-sizing: border-box;
  background: #139dd7;
  color: #fff;
  position: absolute;
  top: 10%;
  right: 5%;
  -moz-transition: ease-in-out 0.5s;
  -o-transition: ease-in-out 0.5s;
  -webkit-transition: ease-in-out 0.5s;
  transition: ease-in-out 0.5s;
}
.lightbox-close:before, .lightbox-close:after {
  content: " ";
  display: block;
  height: 30px;
  width: 1px;
  background: #fff;
  position: absolute;
  left: 26px;
  top: 10px;
  -moz-transform: rotate(45deg);
  -ms-transform: rotate(45deg);
  -webkit-transform: rotate(45deg);
  transform: rotate(45deg);
}
.lightbox-close:after {
  -moz-transform: rotate(-45deg);
  -ms-transform: rotate(-45deg);
  -webkit-transform: rotate(-45deg);
  transform: rotate(-45deg);
}