*{
  font-family: arial;
  padding: 0;
  margin: 0;
}

form {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  
}

.input-field {
  position:relative;
  width:204px;
}

input {
  position: relative;
  
  width: 200px;
  
  margin: 20px;
  padding: 10px 8px;
  
  outline: none;
  border: none;
  border-bottom: 2px  solid black;
  font-size:1rem;
}

input:focus {
  border-bottom: 3px solid black;
}

label {
  position: absolute;
  top: 30%;
  left: 10%;
  
  padding: 5px 10px;
  
  font-size: 1rem;
  
  pointer-events: none;
  transition: all 0.2s ease-in-out 0s;
}

input:not(:invalid) + label,
input:focus + label {
  top: 0;
  
  opacity: 75%;
  font-size: 0.9rem;
  
  background: white;
  
  transition: all .2s ease-in-out;
}

<form>
  <div class="input-field">
    <input type="text" name="forename" required>
    <label>First Name</label>
  </div>

  <div class="input-field">
    <input type="text" name="surname" required>
    <label>Last Name</label>
  </div>
</form>