:invalid

label {
  display: block;
  margin-top: 1em;
}

input:invalid {
  background-color: ivory;
  border: none;
  outline: 2px solid red;
  border-radius: 5px;
}

<form>
  <label for="email">Email Address:</label>
  <input id="email" name="email" type="email" value="na@me@example.com" />

  <label for="secret">Secret Code: (lower case letters)</label>
  <input id="secret" name="secret" type="text" value="test" pattern="[a-z]+" />

  <label for="age">Your age: (18+)</label>
  <input id="age" name="age" type="number" value="5" min="18" />

  <label
    ><input name="tos" type="checkbox" required checked /> - Do you agree to
    ToS?</label
  >
</form>

:invalid {
  /* ... */
}

<form>
  <div class="field">
    <label for="url_input">输入一个 URL：</label>
    <input type="url" id="url_input" />
  </div>

  <div class="field">
    <label for="email_input">输入一个电子邮件地址：</label>
    <input type="email" id="email_input" required />
  </div>
</form>

label {
  display: block;
  margin: 1px;
  padding: 1px;
}

.field {
  margin: 1px;
  padding: 1px;
}

input:invalid {
  background-color: #ffdddd;
}

form:invalid {
  border: 5px solid #ffdddd;
}

input:valid {
  background-color: #ddffdd;
}

form:valid {
  border: 5px solid #ddffdd;
}

input:required {
  border-color: #800000;
  border-width: 3px;
}

input:required:invalid {
  border-color: #c00000;
}

<form>
  <fieldset>
    <label for="form-name">姓名</label><br />
    <input type="text" name="name" id="form-name" required />
  </fieldset>

  <fieldset>
    <label for="form-email">电子邮件</label><br />
    <input type="email" name="email" id="form-email" required />
  </fieldset>

  <fieldset>
    <label for="form-message">信息</label><br />
    <textarea name="message" id="form-message" required></textarea>
  </fieldset>

  <button type="submit" name="send">提交</button>
</form>

/* 在无效的字段集后隐藏该字段集： */
fieldset:invalid ~ fieldset {
  display: none;
}

/* 在表单无效时，将按钮变暗并禁用它： */
form:invalid button {
  opacity: 0.3;
  pointer-events: none;
}

input,
textarea {
  box-sizing: border-box;
  width: 100%;
  font-family: monospace;
  padding: 0.25em 0.5em;
}

button {
  width: 100%;
  border: thin solid darkgrey;
  font-size: 1.25em;
  background-color: darkgrey;
  color: white;
}