HTML 属性: maxlength

<label for="name">Product name:</label>
<input
  id="name"
  name="name"
  type="text"
  value="Shampoo"
  minlength="3"
  maxlength="20"
  required />
<label for="description">Product description:</label>
<textarea
  id="description"
  name="description"
  minlength="10"
  maxlength="40"
  required></textarea>

label {
  display: block;
  margin-top: 1em;
}
input:valid,
textarea:valid {
  background-color: palegreen;
}

<input type="password" maxlength="4" />