Check out example codes for "como fazer placeholder que vira label". It will help you in understanding the concepts better.

Code Example 1

<div class="label-float">
  <input type="text" placeholder=" " />
  <label>Telefone</label>
</div>
<br/>
<div class="label-float">
  <input type="text" placeholder=" " required/>
  <label>Nome de Usuário</label>
</div>

Code Example 2

.label-float {
  position: relative;
  padding-top: 13px;
}

.label-float input {
  border: 1px solid lightgrey;
  border-radius: 5px;
  outline: none;
  min-width: 250px;
  padding: 15px 20px;
  font-size: 16px;
  transition: all .1s linear;
  -webkit-transition: all .1s linear;
  -moz-transition: all .1s linear;
  -webkit-appearance: none;
}

.label-float input:focus {
  border: 2px solid #3951b2;
}

.label-float input::placeholder {
  color: transparent;
}

.label-float label {
  pointer-events: none;
  position: absolute;
  top: calc(50% - 8px);
  left: 15px;
  transition: all .1s linear;
  -webkit-transition: all .1s linear;
  -moz-transition: all .1s linear;
  background-color: white;
  padding: 5px;
  box-sizing: border-box;
}

.label-float input:required:invalid+label {
  color: red;
}

.label-float input:focus:required:invalid {
  border: 2px solid red;
}

.label-float input:required:invalid+label:before {
  content: '*';
}

.label-float input:focus+label,
.label-float input:not(:placeholder-shown)+label {
  font-size: 13px;
  top: 0;
  color: #3951b2;
}

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