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HTML

            
              <div class="container">

  <h1> <span>border-gradient</span> demo</h1>

<div class="circles">
  <div class="circle"></div>
  <div class="circle"></div>
  
  <div class="circle-multiple">
    <div class="circle"></div>
    <div class="circle"></div>
    <div class="circle"></div>
  </div>
</div>
  
  <a class="btn" href="https://twitter.com/exah" target="_blank">
    Follow @exah on Twitter
  </a>

</div>
!

CSS

            
              @import "bourbon";

/// border-gradient mixin: ///

@mixin border-gradient($from, $to, $weight: 0) {
  $mix-main: mix($from, $to);
  $mix-sub-from: mix($mix-main, $from);
  $mix-sub-to: mix($mix-main, $to);
  
  box-shadow: 0 1px 0 $weight rgba($mix-sub-to, .25),
              0 -1px 0 $weight rgba($mix-sub-from, .25),
              1px 0 0 $weight rgba($mix-sub-to, .25),
              -1px 0 0 $weight  rgba($mix-sub-from, .25),
              1px -1px 0 $weight rgba($mix-main, .5),
              -1px 1px 0 $weight rgba($mix-main, .5),
              1px 1px 0 $weight rgba($to, .75),
              -1px -1px 0 $weight rgba($from, .75);
}

/// BASIC EXAMPLE ///

.circle {
  @include size(100px 100px);
  
  border-radius: 100%;
  
  @include border-gradient(red, yellow);
}


////////////// OTHER EXAMPLES ////////////////


.circle {
  &:nth-of-type(2) {
    @include border-gradient(skyblue, hotpink);

    animation: rotateThis 1s linear infinite;
  }

  &:nth-of-type(3) {
    @include border-gradient(#E14847, #B53BB4);
  }
  
  &-multiple { @include size(100px 100px); }
  
  &-multiple & {
    @include position(absolute, 0px 0px 0px 0px);
    margin: auto;
    
    &:nth-of-type(1) { animation: psycho 2s linear infinite; }
    &:nth-of-type(2) { animation: psycho 2s linear .1s infinite; }
    &:nth-of-type(3) { animation: psycho 2s linear .25s infinite; }
  }
 
}

@keyframes rotateThis {
  from {
    transform: rotate(0deg) scale(1);
  }
  
  to {
    transform: rotate(360deg) scale(1);
  }
}

@keyframes psycho {
  0% {
    transform: rotate(0deg) scale(1) translate(0, 0);
  }
  
  33% {
    transform: rotate(360deg) scale(1) translate(5px, 5px);
  }
  
  66% {
    transform: rotate(720deg) scale(1) translate(-5px, -5px);
  }
  
  100% {
    transform: rotate(1080deg) scale(1) translate(0, 0);
  }
}



.btn {
  border: none;
  display: inline-block;
  background: transparent;
  
  color: rgba(white, .7);
  background: rgba(white, .05);
  text-decoration: none;
  
  padding: 15px 20px;
  
  border-radius: 10px;
  
  transition: all .2s;
  
  @include border-gradient(gray, darkgray);
  
  &:hover {
    @include border-gradient(hotpink, skyblue);
    color: white;
    background: rgba(skyblue, .05);
  }
}


///// OTHER /////

.circles > * {
  display: inline-block; vertical-align: top;
  
  position: relative;
  margin: 0 2%;
}

.circles { 
  transform: translateZ(0); 
  margin-bottom: 100px;
}

body {
  background-color: #222; color: #fff;
  text-align: center;
}

.container {
  overflow: hidden;
  padding-bottom: 150px;
}


h1 {
  font: 100 50px/1.25em "Helvetica Neue", Helvetica, Arial, sans-serif;
  margin-bottom: 50px;
  
  span {
    font-weight: 300;
  }
}
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