ARIA in HTML (w3c/html-aria#372) is tightening the allowance of aria-checked on native checkboxes and radios, particularly due to w3c/aria#1622 and the fact that aria-checked=mixed should never be ...

Click on a checkbox (the actual checkbox, not the label or the space between the checkbox and the label) Or: Tab to a checkbox with the keyboard and press space to toggle it. So, this only happens ...