Using the HTML5 "required" attribute for a group of checkboxes?

Unfortunately HTML5 does not provide an out-of-the-box way to do that.

However, using jQuery, you can easily control if a checkbox group has at least one checked element.

Consider the following DOM snippet:

<div class="checkbox-group required">
    <input type="checkbox" name="checkbox_name[]">
    <input type="checkbox" name="checkbox_name[]">
    <input type="checkbox" name="checkbox_name[]">
    <input type="checkbox" name="checkbox_name[]">
</div>

You can use this expression:

$('div.checkbox-group.required :checkbox:checked').length > 0

which returns true if at least one element is checked. Based on that, you can implement your validation check.

HTML5 does not directly support requiring only one/at least one checkbox be checked in a checkbox group. Here is my solution using Javascript:

HTML

<input class='acb' type='checkbox' name='acheckbox[]' value='1' onclick='deRequire("acb")' required> One
<input class='acb' type='checkbox' name='acheckbox[]' value='2' onclick='deRequire("acb")' required> Two

JAVASCRIPT

function deRequireCb(elClass) {
  el = document.getElementsByClassName(elClass);

  var atLeastOneChecked = false; //at least one cb is checked
  for (i = 0; i < el.length; i++) {
    if (el[i].checked === true) {
      atLeastOneChecked = true;
    }
  }

  if (atLeastOneChecked === true) {
    for (i = 0; i < el.length; i++) {
      el[i].required = false;
    }
  } else {
    for (i = 0; i < el.length; i++) {
      el[i].required = true;
    }
  }
}

The javascript will ensure at least one checkbox is checked, then de-require the entire checkbox group. If the one checkbox that is checked becomes un-checked, then it will require all checkboxes, again!

Its a simple trick. This is jQuery code that can exploit the html5 validation by changing the required properties if any one is checked. Following is your html code (make sure that you add required for all the elements in the group.)

<input type="checkbox" name="option[]" id="option-1" value="option1" required/> Option 1
<input type="checkbox" name="option[]" id="option-2" value="option2" required/> Option 2
<input type="checkbox" name="option[]" id="option-3" value="option3" required/> Option 3
<input type="checkbox" name="option[]" id="option-4" value="option4" required/> Option 4
<input type="checkbox" name="option[]" id="option-5" value="option5" required/> Option 5

Following is jQuery script, which disables further validation check if any one is selected. Select using name element.

$cbx_group = $("input:checkbox[name='option[]']");
$cbx_group = $("input:checkbox[id^='option-']"); // name is not always helpful ;)

$cbx_group.prop('required', true);
if($cbx_group.is(":checked")){
  $cbx_group.prop('required', false);
}

Small gotcha here: Since you are using html5 validation, make sure you execute this before the it gets validated i.e. before form submit.

// but this might not work as expected
$('form').submit(function(){
  // code goes here
});

// So, better USE THIS INSTEAD:
$('button[type="submit"]').on('click', function() {
  // skipping validation part mentioned above
});