Pure CSS text input with required tip

index.html

<div class="wrapper">
  <input type="text" required>
  <span class="tip">Required!</span>
</div>

style.css

input:invalid {
  outline: 1px solid red;
}

.tip {
  display: none;
}

input:invalid + .tip {
  display: inline-block;
  color: #fff;
  background-color: #f00;
  margin-left: 10px;
  padding: 2px;
  position: relative;
}

.tip:after {
  position: absolute;
  left: -10px;
  top: 0;
  bottom: 0;
  margin: auto;
  content: '';
  width: 0;
  height: 0;
  border-top: 5px solid transparent;
  border-bottom: 5px solid transparent;
  border-right: 10px solid red;
}