WebForms 2: Validation of checkbox group

For checkboxes, the required attribute shall only be satisfied when one or more of the checkboxes with that name in that form are checked.

Opera 9 and 9.5 beta still follow old version of specification:

For checkboxes, the required attribute shall only be satisfied when the checkbox is checked.

Test

Checkbox group

One or more items should be checked to pass a validation:

Separate checkboxes

All items should be checked to pass a validation: