css増加変数

.button {

  position: relative;

  appearance: none;

  background: #f72359;

  padding: 1em 2em;

  border: none;

  color: white;

  font-size: 1.2em;

  cursor: pointer;

  outline: none;

  overflow: hidden;

  border-radius: 100px;
}


  span {

    position: relative;

  }


  .button:before {

    --size: 0;  


    content: '';

    position: absolute;

    left: var(--x);

    top: var(--y);

    width: var(--size);

    height: var(--size);

    background: radial-gradient(circle closest-side, #4405f7, transparent);

    transform: translate(-50%, -50%);

    transition: width .2s ease, height .2s ease;

  }


  .button:hover:before {

    --size: 400px;

  }

document.querySelector('.button').onmousemove = (e) => {


  const x = e.pageX-e.target.offsetLeft;

  const y = e.pageY-e.target.offsetTop;


  e.target.style.setProperty('--x', `${ x }px`)

  e.target.style.setProperty('--y', `${ y }px`)
}