<!-- not working in safari or ios -->
<h1 id="counter"><span></span><b></b><i></i></h1>

html,
body {
  margin: 0;
  padding: 0;
}

body {
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
}
body:before {
  content: "";
  position: fixed;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
  z-index: -1;
  background: linear-gradient(red, red);
  background-size: 100% 0;
  background-repeat: no-repeat;
  animation: bg 5s linear forwards;
}
#counter {
  font-family: monospace;
  margin: 0;
  text-align: center;
  padding: 1rem 2rem;
  border: 5px solid #000;
  letter-spacing: 5px;
  font-size: 4rem;
  background: #fff;
  box-shadow: 0 0 20px 20px rgba(0, 0, 0, 0.5);
  min-width: 8rem;
  background: linear-gradient(#ccc, #eee);
  mix-blend-mode: luminosity;
}
h1 * {
  font-weight: bold;
  font-style: normal;
}
@keyframes bg {
  100% {
    background-size: 100% 100%;
  }
}
#counter i:before {
  content: "0";
  animation: units 0.5s linear forwards;
  animation-iteration-count: 10;
}
@keyframes units {
  0% {
    content: "0";
  }
  10% {
    content: "1";
  }
  20% {
    content: "2";
  }
  30% {
    content: "3";
  }
  40% {
    content: "4";
  }
  50% {
    content: "5";
  }
  60% {
    content: "6";
  }
  70% {
    content: "7";
  }
  80% {
    content: "8";
  }
  90% {
    content: "9";
  }
  100% {
    content: "0";
  }
}

#counter b:before {
  content: "0";
  animation: tens 5s linear forwards;
  animation-iteration-count: 1;
}
@keyframes tens {
  0% {
    content: "0";
  }
  9% {
    content: "0";
  }
  10% {
    content: "1";
  }
  19% {
    content: "1";
  }
  20% {
    content: "2";
  }
  29% {
    content: "2";
  }
  30% {
    content: "3";
  }
  39% {
    content: "3";
  }
  40% {
    content: "4";
  }
  49% {
    content: "4";
  }
  50% {
    content: "5";
  }
  59% {
    content: "5";
  }
  60% {
    content: "6";
  }
  69% {
    content: "6";
  }
  70% {
    content: "7";
  }
  79% {
    content: "7";
  }
  80% {
    content: "8";
  }
  89% {
    content: "8";
  }
  90% {
    content: "9";
  }
  99% {
    content: "9";
  }
  100% {
    content: "0";
  }
}

#counter span:before {
  content: "0";
  animation: hundreds 5s linear forwards;
  animation-iteration-count: 1;
}
@keyframes hundreds {
  0% {
    content: "0";
  }
  99% {
    content: "0";
  }
  100% {
    content: "1";
  }
}