.a {
  list-style-type: none;
  margin: 0;
  padding: 0;
  margin-left: auto;
  margin-right: auto;
  width: 80%;
  position: relative;
}

.c {
  width: calc( 100% / 6 );
  height: 150px;
  display: inline-block;
  margin-left: -5%;
  -webkit-animation: one 13s infinite;
          animation: one 13s infinite;
}

.c:nth-of-type(even) {
  -webkit-animation: two 17s infinite;
          animation: two 17s infinite;
}

.c:nth-of-type(1) {
  background-color: red;
  margin-left: 0;
}

.c:nth-of-type(2) {
  background-color: green;
}

.c:nth-of-type(3) {
  background-color: blue;
}

.c:nth-of-type(4) {
  background-color: cyan;
}

.c:nth-of-type(5) {
  background-color: magenta;
}

.c:nth-of-type(6) {
  background-color: yellow;
}

@-webkit-keyframes one {
  0% {
    opacity: 0.9;
  }
  50% {
    opacity: 0.3;
  }
  100% {
    opacity: 0.9;
  }
}

@keyframes one {
  0% {
    opacity: 0.9;
  }
  50% {
    opacity: 0.3;
  }
  100% {
    opacity: 0.9;
  }
}
@-webkit-keyframes two {
  0% {
    opacity: 0.2;
  }
  50% {
    opacity: 0.9;
  }
  100% {
    opacity: 0.2;
  }
}
@keyframes two {
  0% {
    opacity: 0.2;
  }
  50% {
    opacity: 0.9;
  }
  100% {
    opacity: 0.2;
  }
}