Анимация иконки загрузки при помощи CSS

Самый простой круглый значок.

Его можно сделать с помощью background и animation

<style class="style" contenteditable="">
.loading {
  position: relative;
  width: 0;
  padding: 20px;  /* если задать в %, то будет рассчитываться от ширины родителя */
  background:
   linear-gradient(rgba(0,0,0,1) 30%, transparent 30%, transparent 70%, rgba(0,0,0,.4) 70%),
   linear-gradient(to left, rgba(0,0,0,.2) 30%, transparent 30%, transparent 70%, rgba(0,0,0,.8) 70%);
  background-repeat: no-repeat;
  background-size: 10% 100%, 100% 10%;
  background-position: 50% 0%, 0 50%;
  -webkit-animation: loading .7s infinite steps(8);
  animation: loading .7s infinite steps(8);
}
.loading:after {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-image:
   linear-gradient(rgba(0,0,0,.1) 30%, transparent 30%, transparent 70%, rgba(0,0,0,.5) 70%),
   linear-gradient(to left, rgba(0,0,0,.3) 30%, transparent 30%, transparent 70%, rgba(0,0,0,.9) 70%);
  background-repeat: no-repeat;
  background-size: 10% 100%, 100% 10%;
  background-position: 50% 0%, 0 50%;
  -webkit-transform: rotate(45deg);
  transform: rotate(45deg);
}
@-webkit-keyframes loading {
  100% {-webkit-transform: rotate(1turn);}
}
@keyframes loading {
  100% {transform: rotate(1turn);}
}

<div class="loading"></div>

<style class="style" contenteditable="">
.loading1 {
  width: 0;
  padding: 12px;
  background-image:
   radial-gradient(circle at 0% 0%, transparent 40%, rgb(0,255,0) 40%, rgb(0,255,0) 70%, transparent 70%),
   radial-gradient(circle at 100% 0%, transparent 40%, rgba(0,255,0,.2) 40%, rgba(0,255,0,.2) 70%, transparent 70%),
   radial-gradient(circle at 100% 100%, transparent 40%, rgba(0,255,0,.2) 40%, rgba(0,255,0,.2) 70%, transparent 70%),
   radial-gradient(circle at 0% 100%, transparent 40%, rgba(0,255,0,.2) 40%, rgba(0,255,0,.2) 70%, transparent 70%);
  background-repeat: no-repeat;
  background-size: 50% 50%;
  background-position: 100% 100%, 0 100%, 0 0, 100% 0;
  -webkit-animation: loading1 linear .7s infinite;
  animation: loading1 linear .7s infinite;
}
@-webkit-keyframes loading1 {
  100% {-webkit-transform: rotate(360deg);}
}
@keyframes loading1 {
  100% {transform: rotate(360deg);}
}
</style>
<div class="loading1"></div>

Или создать благодаря border и animation

<style class="style" contenteditable="">
.loading2 {
  width: 15px;
  height: 15px;
  border: 5px solid #ccffcc;  /* полупрозрачный цвет ставить не стоит, появляются ненужные линии */
  border-top-color: rgb(0,255,0);
  border-radius: 100%;
  -webkit-animation: loading2 linear .7s infinite;
  animation: loading2 linear .7s infinite;
}
@-webkit-keyframes loading2 {
  100% {-webkit-transform: rotate(360deg);}
}
@keyframes loading2 {
  100% {transform: rotate(360deg);}
}
</style>
<div class="loading2"></div>