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 able to be exposed by browsers that correctly follow the ARIA spec.

we would request that this at the very least this be called out as an author error if using aria-checked=mixed on a checkbox, and either an author error or a warning to indicate that authors should not use this attribute on native checkboxes and radios.

For instance, there is a presently bug in chromium where aria-checked incorrectly takes precedent over the native checked / dirty checked state of an radio or checkbox element, which can cause inaccurate information to be exposed if the actual checked state and the aria-checked attribute are out of alignment: https://bugs.chromium.org/p/chromium/issues/detail?id=1260371