ARIA in HTML is updating it's allowances for aria-checked on native checkboxes and radio buttons.

See:
w3c/html-aria#372 and related ARIA issue w3c/aria#1622

This is particularly problematic for aria-checked=mixed on native checkbox elements, as aria-checked must be ignored, regardless of its value. Instead authors need to use the indeterminate IDL attribute to indicate the 'mixed' state, with native HTML checkbox elements.

We would request the HTML validator update to indicate use of aria-checked on radio and checkboxes to be an error, rather than a warning.