<h1>Spotlight!</h1>
<button class="reveal">Remove Spotlight...</button>

<div class="spotlight"></div>

@import url('https://fonts.googleapis.com/css2?family=Tagesschrift&display=swap');

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  font-family: "Tagesschrift", system-ui;
  min-height: 100vh;
  display: grid;
  place-items: center;
  align-content: center;
  background-image: linear-gradient(to bottom right, aquamarine, hotpink);
  color: white;
}

h1 {
  font-size: 8em;
}

button {
  font: inherit;
  padding: 1em 2em;
  background: none;
  color: #777;
  border: 2px solid;
  border-radius: 4em;
  transition: color 0.4s;

  &:hover { 
    color: #fff;
  }
}

.spotlight {
  position: fixed;
  inset: 0;
  opacity: 0.95;
  pointer-events: none;
  filter: blur(0.1em) contrast(10);
  mix-blend-mode: darken;
  background-color: white;
  outline: 2em solid black;
  background-image:
    radial-gradient(circle at var(--clientX, 50%) var(--clientY, 50%), transparent, black 14em),
    repeating-linear-gradient(0deg, black 0, transparent 0.1em 2.9em, black 3em),
    repeating-linear-gradient(90deg, black 0, transparent 0.1em 2.9em, black 3em);
  background-position: 0 0, 0 calc(var(--clientY, 50%) * -0.2), calc(var(--clientX, 50%) * -0.2) 0;
  transition: opacity 1s, background-color 1s;

  body:has(.reveal:hover) & {
    opacity: 0;
    background-color: black;
  }
}

document.body.addEventListener('mousemove', (e) => {
  document.body.style.setProperty('--clientX', e.clientX + 'px');
  document.body.style.setProperty('--clientY', e.clientY + 'px');
});