:root {
  --borderWidth: 10px;
  --borderRadius: 10px;
  --width: 400px;
  --height: 225px;
  --hue: 277;
  --hueOffset: 120;
  --level: 50%;
  --brightness: 100%;
  --gradient: radial-gradient(
    circle,
    hsl(var(--hue), 100%, var(--brightness)) 0%,
    hsl(calc(var(--hue) + var(--hueOffset)), 100%, var(--level)) 100%
  );
  --duration: 5s;
  --fraction: calc(var(--width) / (var(--width) + var(--height)));
}
.gradient-border-box {
  width: 100%;
  height: auto;
  border-radius: var(--borderRadius);
  -webkit-mask: -webkit-gradient(
        linear,
        left top,
        left bottom,
        from(#fff),
        to(#fff)
      )
      top,
    -webkit-gradient(linear, left top, left bottom, from(#fff), to(#fff)) bottom,
    -webkit-gradient(linear, left top, left bottom, from(#fff), to(#fff)) left,
    -webkit-gradient(linear, left top, left bottom, from(#fff), to(#fff)) right;
  -webkit-mask: linear-gradient(#fff, #fff) top,
    linear-gradient(#fff, #fff) bottom, linear-gradient(#fff, #fff) left,
    linear-gradient(#fff, #fff) right;
  mask: -webkit-gradient(linear, left top, left bottom, from(#fff), to(#fff))
      top,
    -webkit-gradient(linear, left top, left bottom, from(#fff), to(#fff)) bottom,
    -webkit-gradient(linear, left top, left bottom, from(#fff), to(#fff)) left,
    -webkit-gradient(linear, left top, left bottom, from(#fff), to(#fff)) right;
  mask: linear-gradient(#fff, #fff) top, linear-gradient(#fff, #fff) bottom,
    linear-gradient(#fff, #fff) left, linear-gradient(#fff, #fff) right;
  -webkit-mask-size: 100% var(--borderWidth), 100% var(--borderWidth),
    var(--borderWidth) 100%, var(--borderWidth) 100%;
  mask-size: 100% var(--borderWidth), 100% var(--borderWidth),
    var(--borderWidth) 100%, var(--borderWidth) 100%;
  -webkit-mask-repeat: no-repeat;
  mask-repeat: no-repeat;
  display: none;
}

.gradient-border {
  background: transparent;
  position: relative;
}

.gradient-border:after {
  content: "";
  position: absolute;
  top: calc(-1 * var(--borderWidth));
  left: calc(-1 * var(--borderWidth));
  height: calc(100% + var(--borderWidth) * 2);
  width: calc(100% + var(--borderWidth) * 2);
  background: var(--gradient);
  border-radius: calc(2 * var(--borderRadius));
  -webkit-animation: animatedgradient var(--duration) ease-in-out infinite;
  animation: animatedgradient var(--duration) ease-in-out infinite;
  background-size: 200% 200%;
}

@-webkit-keyframes animatedgradient {
  0% {
    background-position: 0% 0%;
  }
  calc(0.5 * var(--fraction)) {
    background-position: 100% 0%;
  }
  50% {
    background-position: 100% 100%;
  }
  calc(0.5 + 0.5 * var(--fraction)) {
    background-position: 0% 100%;
  }
  100% {
    background-position: 0% 0%;
  }
}

@keyframes animatedgradient {
  0% {
    background-position: 0% 0%;
  }
  25% {
    background-position: 100% 0%;
  }
  50% {
    background-position: 100% 100%;
  }
  75% {
    background-position: 0% 100%;
  }
  100% {
    background-position: 0% 0%;
  }
}