Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="spinner asset-detail-preview__spinner"></div>

SassMeister-input.scss

// ----
// Sass (v3.4.13)
// Compass (v1.0.3)
// ----

@mixin spinner($color) { 

  min-width: 24px;
  min-height: 24px;

  &:before {
    content: 'Loading…';
    position: absolute;
    top: 50%;
    left: 50%;
    width: 16px;
    height: 16px;
    margin-top: -10px;
    margin-left: -10px;
  }

  &:not(:required):before {
    content: '';
    border-radius: 50%;
    border: 2px solid rgba($color, .3);
    border-top-color: rgba($color, .6);
    animation: spinner .6s linear infinite;
    -webkit-animation: spinner .6s linear infinite;
  }

  @keyframes spinner {
    to {transform: rotate(360deg);}
  }

  @-webkit-keyframes spinner {
    to {-webkit-transform: rotate(360deg);}
  }
}

.asset-detail-preview__spinner {
  @include spinner(blue);
}

SassMeister-output.css

@charset "UTF-8";
.asset-detail-preview__spinner {
  min-width: 24px;
  min-height: 24px;
}
.asset-detail-preview__spinner:before {
  content: 'Loading…';
  position: absolute;
  top: 50%;
  left: 50%;
  width: 16px;
  height: 16px;
  margin-top: -10px;
  margin-left: -10px;
}
.asset-detail-preview__spinner:not(:required):before {
  content: '';
  border-radius: 50%;
  border: 2px solid rgba(0, 0, 255, 0.3);
  border-top-color: rgba(0, 0, 255, 0.6);
  animation: spinner .6s linear infinite;
  -webkit-animation: spinner .6s linear infinite;
}
@keyframes spinner {
  to {
    transform: rotate(360deg);
  }
}
@-webkit-keyframes spinner {
  to {
    -webkit-transform: rotate(360deg);
  }
}

SassMeister-rendered.html

<div class="spinner asset-detail-preview__spinner"></div>