<div class="container">

  <h1 class="text-center">
    CSS only Input Label animation
  </h1>

  <div class="row">
    <div class="col-8 col-md-6">
      <div class="form-group">
        <input id="name1" type="text" placeholder="Please enter something" required>
        <label for="name1">Please enter something</label>
      </div>
      <div class="form-group">
        <input id="name2" type="text" pattern=".{6,}" placeholder="Please enter something" required>
        <label for="name2">Please enter at least 6 characters</label>
      </div>
      <div class="form-group">
        <input id="name3" type="email" placeholder="Please enter something" required pattern="[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,}$">
        <label for="name3">Please enter email</label>
      </div>
    </div>
  </div>
</div>
.row {
  justify-content: center;
  align-items: center;
}
.form-group {
  margin: 2rem 0;
  position:relative;
}
label {
  position:absolute;
  top:50%;
  left:1rem;
  transform: translate3d(0,-50%,0);
  margin:0;
  padding: 0;
  transition: all .4s cubic-bezier(.05,.88,.36,.99);
    opacity:0.5;
}
input {
  display: block;
  width: 100%;
  padding: 0.75em;
  margin:0;
  box-sizing: border-box;
  border: 1px solid silver;
  border-radius: 4px;
  background-color: transparent;
  &::-webkit-input-placeholder {
    color:transparent;
  }
  &:hover,
  &:focus {
    box-shadow: 0 1px 4px -2px rgba(0, 0, 0, 1);
    outline: none;
  }
  &:not(:placeholder-shown) ~ label {
    top:-1em;
    left:0;
    font-size:75%;
  }
  &:valid {
      border-color: limegreen;
  }
  &:not(:placeholder-shown):not(:valid) {
      border-color: red;
  }
}
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External CSS

  1. https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.3/css/bootstrap.min.css

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