Cross-Fade between images with CSS in loop

I want to fade between images in a loop (like result here-jsfiddle.net/5M2PD) but purely through CSS, no JavaScript. I tried using key-frames but I wasn't successful. Please Help.

@keyframes cf3FadeInOut {
    0% {
        opacity:1;
    }
    45% {
        opacity:1;
    }
    55% {
        opacity:0;
    }
    100% {
        opacity:0;
    }
}

#cf3 img.top {
   animation-name: cf3FadeInOut;
   animation-timing-function: ease-in-out;
   animation-iteration-count: infinite;
   animation-duration: 10s;
   animation-direction: alternate;
}

I have taken your fiddle as a base, and made it work without script.

updated demo

I needed to set an id to the HTML

.fadein img {
    position:absolute;
    top:0;
    -webkit-animation-name: fade;
    -webkit-animation-iteration-count: infinite;
    -webkit-animation-duration: 6s;
    animation-name: fade;
    animation-iteration-count: infinite;
    animation-duration: 6s;
}

@-webkit-keyframes fade {
    0% {opacity: 0;}
    20% {opacity: 1;}
    33% {opacity: 1;}
    53% {opacity: 0;}
    100% {opacity: 0;}
}
@keyframes fade {
    0% {opacity: 0;}
    20% {opacity: 1;}
    33% {opacity: 1;}
    53% {opacity: 0;}
    100% {opacity: 0;}
}

#f1 {
    background-color: lightblue;
}
#f2 {
    -webkit-animation-delay: -4s;
    background-color: yellow;
}
#f3 {
    -webkit-animation-delay: -2s;
    background-color: lightgreen;
}

<div class="fadein">
    <img id="f3" src="http://i.imgur.com/R7A9JXc.png">
    <img id="f2" src="http://i.imgur.com/D5yaJeW.png">
    <img id="f1" src="http://i.imgur.com/EUqZ1Er.png">
</div>

I am setting the keyframes to give aprox 1/3 of the time visible, with apropiate transitions. Then I set different delays for every image, so that they alternate. If you want full browser support, you will need more vendor prefixes. I have used -webkit- and bare property so that you get the idea.

I've made a dynamic solution with SASS. It's possible to configure:

• Total animation time
• Amount of items
• Transition speed

It automatically calculates the keyframe percentages and delays between items.

Codepen.io demo

// Demo styles
.fadecycle div {
  opacity: 0;
  position: absolute;
  width: 200px;
  line-height: 200px;
  text-align: center;
}

.fadecycle div:nth-child(1) { background: lightsalmon; }
.fadecycle div:nth-child(2) { background: lightsteelblue; }
.fadecycle div:nth-child(3) { background: lightseagreen; }
.fadecycle div:nth-child(4) { background: lightskyblue; }

// Animation settings
$totalTime: 8s;
$items: 4;
$transitionSpeed: 1.5;

// Calculate transition + display time in seconds
$transitionTime: 0s + $totalTime / ($items * $transitionSpeed * 2);
$displayTime: (0s + $totalTime - ($transitionTime * $items)) / $items;

// Set transition for each element
@for $i from 1 through $items {
  .fadecycle div:nth-child(#{$i}) {
    // Delay is increased for each item
    // starting with an offset of -$transitionTime so the first element is displayed on load
    $delay: -$transitionTime + ($transitionTime + $displayTime) * ($i - 1);
    animation: fadeinout $totalTime linear $delay infinite;
  }
}

// Calculate percentages of the times for keyframes
$transitionPercentage: 0% + 100 * ($transitionTime / $totalTime);
$displayPercentage: 0% + 100 * ($displayTime / $totalTime);

@keyframes fadeinout {
  0% {
    opacity: 0;
  }
  #{$transitionPercentage},
  #{$transitionPercentage + $displayPercentage} {
    opacity: 1;
  }
  #{$transitionPercentage + $displayPercentage + $transitionPercentage},
  100% {
    opacity: 0;
  }
}