<title>BUTTONS ANIMATION</title>

<a class="box__link button-animation" href="#">
  Submit
  <span></span>
  <span></span>
  <span></span>
  <span></span>
</a>

/*=============== GOOGLE FONTS ===============*/
@import url("https://fonts.googleapis.com/css2?family=Montserrat:wght@400;600&display=swap");

/*=============== VARIABLES CSS ===============*/
:root {
  /*========== Colors ==========*/
  /*Color mode HSL(hue, saturation, lightness)*/
  --body-color: #393937;
  --accent-color: #03e9f4;
  --white-color: #fff;
  --box-shadow: 0 0 5px #03e9f4, 0 0 25px #03e9f4, 0 0 50px #03e9f4,
    0 0 100px #03e9f4;

  /*========== Font and typography ==========*/
  /*.5rem = 8px | 1rem = 16px ...*/
  --body-font: "Montserrat", sans-serif;
  --normal-font-size: 1rem;

  /*========== Font weight ==========*/
  --font-regular: 400;
  --font-semi-bold: 600;

  /*========== z index ==========*/
  --z-tooltip: 10;
  --z-fixed: 100;
}

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

body {
  font-family: var(--body-font);
  font-size: var(--normal-font-size);
  background: var(--body-color);
  min-height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
}

ul {
  list-style: none;
}

a {
  text-decoration: none;
}

.button-animation {
  position: relative;
  display: inline-block;
  padding: 20px 30px;
  color: var(--accent-color);
  text-transform: uppercase;
  overflow: hidden;
  letter-spacing: 4px;
  transition: 0.5s;
  span {
    position: absolute;
    display: block;
  }

  span:nth-child(1) {
    top: 0;
    left: -100%;
    width: 100%;
    height: 2px;
    background: linear-gradient(90deg, transparent, var(--accent-color));
    animation: btn-anim1 1s linear infinite;
  }
  @keyframes btn-anim1 {
    0% {
      left: -100%;
    }
    50%,
    100% {
      left: 100%;
    }
  }

  span:nth-child(2) {
    top: -100%;
    right: 0;
    width: 2px;
    height: 100%;
    background: linear-gradient(180deg, transparent, var(--accent-color));
    animation: btn-anim2 1s linear infinite;
    animation-delay: 0.25s;
  }
  @keyframes btn-anim2 {
    0% {
      top: -100%;
    }
    50%,
    100% {
      top: 100%;
    }
  }

  span:nth-child(3) {
    bottom: 0;
    right: -100%;
    width: 100%;
    height: 2px;
    background: linear-gradient(270deg, transparent, var(--accent-color));
    animation: btn-anim3 1s linear infinite;
    animation-delay: 0.5s;
  }
  @keyframes btn-anim3 {
    0% {
      right: -100%;
    }
    50%,
    100% {
      right: 100%;
    }
  }

  span:nth-child(4) {
    bottom: -100%;
    left: 0;
    width: 2px;
    height: 100%;
    background: linear-gradient(360deg, transparent, var(--accent-color));
    animation: btn-anim4 1s linear infinite;
    animation-delay: 0.75s;
  }

  @keyframes btn-anim4 {
    0% {
      bottom: -100%;
    }
    50%,
    100% {
      bottom: 100%;
    }
  }

  &:hover {
    background-color: var(--accent-color);
    color: var(--white-color);
    border-radius: 5px;
    box-shadow: var(--box-shadow);
  }
}