Transitions

transitions.css

a {
  color: gray;
  -webkit-transition: color 0.3s ease;
  -moz-transition: color 0.3s ease;
  -ms-transition: color 0.3s ease;
  -o-transition: color 0.3s ease;
  transition: color 0.3s ease;
}
a:hover {
  color: black;
}

transitions.scss

// ----
// Sass (v3.4.7)
// Compass (v1.0.1)
// ----

@mixin transition($args...) {
  -webkit-transition: $args;
  -moz-transition: $args;
  -ms-transition: $args;
  -o-transition: $args;
  transition: $args;
}

// Usage

a {
  color: gray;
  @include transition(color .3s ease);
  &:hover {
    color: black;
  }
}