*{
  box-sizing:border-box;
  margin:0;
  padding:0;
}
body {
  height:100vh;
  display:flex;
  align-items:center;
  justify-content:center;
  background-color: #212121;
  font-family:arial;
}
.form__group {
  position: relative;
  padding: 20px 0 0;
  width: 100%;
  max-width: 180px;
}

.form__field {
  font-family: inherit;
  width: 100%;
  border: none;
  border-bottom: 2px solid #9b9b9b;
  outline: 0;
  font-size: 17px;
  color: #fff;
  padding: 7px 0;
  background: transparent;
  transition: border-color 0.2s;
}

.form__field::placeholder {
  color: transparent;
}

.form__field:placeholder-shown ~ .form__label {
  font-size: 17px;
  cursor: text;
  top: 20px;
}

.form__label {
  position: absolute;
  top: 0;
  display: block;
  transition: 0.2s;
  font-size: 17px;
  color: #9b9b9b;
  pointer-events: none;
}

.form__field:focus {
  padding-bottom: 6px;
  font-weight: 700;
  border-width: 3px;
  border-image: linear-gradient(to right, #116399, #38caef);
  border-image-slice: 1;
}

.form__field:focus ~ .form__label {
  position: absolute;
  top: 0;
  display: block;
  transition: 0.2s;
  font-size: 17px;
  color: #38caef;
  font-weight: 700;
}

.form__field:required, .form__field:invalid {
  box-shadow: none;
}