<h1 id="counter"></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;
}
h1 {
  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;
}
.startBG:before {
  animation: bg 5s linear forwards;
}
@keyframes bg {
  100% {
    background-size: 100% 100%;
  }
}

(function (d) {
  "use strict";
  const myElement = d.getElementById("counter");
  d.body.classList.add("startBG");
  var counter = 0;
  var countdown = setInterval(function () {
    counter++;
    myElement.textContent = counter;
    if (counter == 100) clearInterval(countdown);
  }, 50);
})(document);