<div class="box">content inside</div>
<!-- Inspired by the mega buttons on https://turbo.build/ -->

html {
  height: 100%;
  background: #223;
  display: grid;
  place-items: center;
}

.box {
  width: 60vmin;
  height: 50vmin;
  display: grid;
  place-content: center;
  color: white;
  text-shadow: 0 1px 0 #000;
  
  --border-angle: 0turn; // For animation.
  --main-bg: conic-gradient(
      from var(--border-angle),
      #213,
      #112 5%,
      #112 60%,
      #213 95%
    );
  
  border: solid 5px transparent;
  border-radius: 2em;
  --gradient-border: conic-gradient(from var(--border-angle), transparent 25%, #08f, #f03 99%, transparent);
  
  background: 
    // padding-box clip this background in to the overall element except the border.
    var(--main-bg) padding-box,
    // border-box extends this background to the border space
    var(--gradient-border) border-box, 
    // Duplicate main background to fill in behind the gradient border. You can remove this if you want the border to extend "outside" the box background.
    var(--main-bg) border-box;
  
  background-position: center center;

  animation: bg-spin 3s linear infinite;
  @keyframes bg-spin {
    to {
      --border-angle: 1turn;
    }
  }

  &:hover {
    animation-play-state: paused;
  }
}

@property --border-angle {
  syntax: "<angle>";
  inherits: true;
  initial-value: 0turn;
}