表单 – 在表单的占位符中添加span

.input-placeholder {
  position: relative;
}
.input-placeholder input {
  padding: 10px;
  font-size: 25px;
}
.input-placeholder input:valid + .placeholder {
  display: none;
}

.placeholder {
  position: absolute;
  pointer-events: none;
  top: 0;
  bottom: 0;
  height: 25px;
  font-size: 25px;
  left: 10px;
  margin: auto;
  color: #ccc;
}

.placeholder span {
  color: red;
}

<form novalidate>
    <div class="input-placeholder">
        <input type="text" required>
        <div class="placeholder">
            Email <span>*</span>
        </div>
    </div>
    <input type="submit">
</form>