Animated Checkbox & Label with Pure CSS/CSS3

Preview:

Description:

Adding smooth check/uncheck transition effects to native checkboxes using CSS3 transitions, animations, transforms and @keyframes.

How to use it:

Add prefixfree.min.js for CSS3 vendor prefixes.

<script src="prefixfree.min.js"></script>

Create a normal checkbox with text label.

<input id="demo" type="checkbox">
<label for="demo">
  <span></span>
  Lorem ipsum dolor sit amet
  <ins><i>Lorem ipsum dolor sit amet</i></ins>
</label>

Reset CSS.

* {
  box-sizing: border-box;
  user-select: none;
}

The core CSS/CSS3 rules.

input[type='checkbox'] {
  height: 0;
  width: 0;
}
input[type='checkbox'] + label {
  position: relative;
  display: flex;
  margin: .6em 0;
  align-items: center;
  color: #9e9e9e;
  transition: color 250ms cubic-bezier(.4, .0, .23, 1);
}
input[type='checkbox'] + label > ins {
  position: absolute;
  display: block;
  bottom: 0;
  left: 2em;
  height: 0;
  width: 100%;
  overflow: hidden;
  text-decoration: none;
  transition: height 300ms cubic-bezier(.4, .0, .23, 1);
}
input[type="checkbox"] + label > ins > i {
  position: absolute;
  bottom: 0;
  font-style: normal;
  color: #4FC3F7;
}
input[type="checkbox"] + label > span {
  display: flex;
  justify-content: center;
  align-items: center;
  margin-right: 1em;
  width: 1em;
  height: 1em;
  background: transparent;
  border: 2px solid #9E9E9E;
  border-radius: 2px;
  cursor: pointer;
  transition: all 250ms cubic-bezier(.4, .0, .23, 1);
}
input[type="checkbox"] + label:hover,
input[type="checkbox"]:focus + label { color: #fff; }
input[type="checkbox"] + label:hover > span,
input[type="checkbox"]:focus + label > span { background: rgba(255,255,255,.1); }
input[type="checkbox"]:checked + label > ins { height: 100%; }
input[type="checkbox"]:checked + label > span {
  border: .5em solid #FFEB3B;
  animation: shrink-bounce 200ms cubic-bezier(.4, .0, .23, 1);
}
input[type="checkbox"]:checked + label > span:before {
  content: "";
  position: absolute;
  top: .6em;
  left: .2em;
  border-right: 3px solid transparent;
  border-bottom: 3px solid transparent;
  transform: rotate(45deg);
  transform-origin: 0% 100%;
  animation: checkbox-check 125ms 250ms cubic-bezier(.4, .0, .23, 1) forwards;
}

Create the animations using CSS3 @keyframes.

@keyframes 
shrink-bounce {  0% {
 transform: scale(1);
}
 33% {
 transform: scale(.85);
}
 100% {
 transform: scale(1);
}
}
@keyframes 
checkbox-check {  0% {
 width: 0;
 height: 0;
 border-color: #212121;
 transform: translate3d(0, 0, 0) rotate(45deg);
}
 33% {
 width: .2em;
 height: 0;
 transform: translate3d(0, 0, 0) rotate(45deg);
}
 100% {
 width: .2em;
 height: .5em;
 border-color: #212121;
 transform: translate3d(0, -.5em, 0) rotate(45deg);
}
}