按顺序启用表单中的字段集

document.querySelectorAll('fieldset').forEach(fs => {
  fs.addEventListener('change', function() {
    var next = this.nextElementSibling;
    
    while (next && !next.disabled) {
      next = next.nextElementSibling;
    }
    
    if (next) {
      next.disabled = false;
    }
  });
})

<fieldset id="foo">
  <label><input type="radio" name="foo">Apples</label>
  <label><input type="radio" name="foo">Oranges</label>
</fieldset>

<fieldset id="bar" disabled>
  <label><input type="radio" name="bar">Bus</label>
  <label><input type="radio" name="bar">Airplane</label>
</fieldset>

<fieldset id="bar" disabled>
  <label><input type="radio" name="x">Bus1</label>
  <label><input type="radio" name="x">Airplane1</label>
</fieldset>

<fieldset id="bar" disabled>
  <label><input type="radio" name="y">Bus2</label>
  <label><input type="radio" name="y">Airplane2</label>
</fieldset>