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                <form id="form">
    <input type="text" name="_text" required/>
  <button id="submit" type="submit" disabled>送信</button>


select {
  &:valid {
    border: 1px solid green;
  &:invalid {
    border: 1px solid red;

// 案件上デフォルトのstyleで使われることがほぼないのでstyleは適当
[type='checkbox'] {
  &:valid {
    box-shadow: green 0px 0px 3px 1px;
  &:invalid {
    box-shadow: red 0px 0px 3px 1px;

small {
  color: red;


                const $form = document.getElementById('form');
const $submit = document.getElementById('submit');

// 親要素(form)にイベントをつけると子要素のイベントが拾える
$form.addEventListener('change', update);
$form.addEventListener('input', update);

// デフォルトではEnterキー押下でも送信されてしまうのでsubmitイベントで制御する
$form.addEventListener('submit', (e) => {
  // $form.submit(); or ajaxする など

function update(e) {
  // バリデーションが総合的に通っているかどうかのフラグ
  const isValid = $form.checkValidity();
  if (isValid) {  // 通っていれば活性化
  // 通っていなければ非活性にする
  $submit.setAttribute('disabled', 'disabled');