<div class="input-with-placeholder">
    <input type="text" id="username" required />
    <label for="username">Username</label>
</div>

* {
  padding: 0;
  margin: 0;
  list-style: none;
  border: 0;
  box-sizing: border-box;
  font-family: Arial, sans-serif;
  outline: 0;
}

body {
  padding: 40px;
}

.input-with-placeholder {
  width: 250px;
  position: relative;
  label {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    display: flex;
    align-items: center;
    padding-left: 10px;
    transition: 300ms all;
  }
  input {
    width: 100%;
    height: 40px;
    padding: 0 15px;
    background: #f8f8f8;
    &:valid + label,
    &:focus + label {
      height: 50%;
      padding-left: 0;
      transform: translateY(-100%);
      font-size: 12px;
      color: #777;
    }
  }
}