This change will make it extremely difficult for implementors to implement checkboxes that are styled AND are accessible. Currently most component libraries that implement accessible styled checkboxes use a real checkbox elements for the focus but overlay a visibly styled element of some sort.

I think that this is a change that takes the web platform in the wrong direction.

Currently most component libraries that implement accessible styled checkboxes use a real checkbox elements for the focus but overlay a visibly styled element of some sort.

That is true

This change will make it extremely difficult for implementors to implement checkboxes that are styled AND are accessible

This is not true as the frameworks do not require ARIA. There is the invisible HTML checkbox (without ARIA) and the visible checkbox (which is marked with aria-hidden=true at most, depending on the implementation)

This update is being made to better reflect reality, and to inform authors that what they are doing now is not consistently accessible. Using aria-checked on native checkboxes and radio buttons is not respected in Firefox and Safari, which includes the aria-checked=mixed state. These browsers are correctly following the ARIA spec, as it has been written since version 1.0. Native attributes win out over aria-* equivalents.

Authors using aria-checked=mixed on a native checkbox are presently only exposing this in Chromium browsers, due to their incorrect implementation of ARIA.

Authors should be using the indeterminate IDL attribute to set their checkboxes to the mixed state - which per standard visually hidden checkbox techniques, would work similarly to what authors are doing with their [aria-checked=mixed] + span selectors now.

Further discussion on whether aria-checked needs to be a special case (because it actually is compared to other aria-* attributes, since it combines the checkboxes native checked and indeterminate attributes into one aria-* attribute) can occur in w3c/aria#1622.

verified these rules are included in the IBM Equal Access preview rules. This now meets the 2 implementations needed to be merged into the spec.