The Attribute Isn't the Behavior
Open any component library's Dialog implementation. You'll find role="dialog" and aria-modal="true" on the content panel. Check the box, ship it, call it accessible.
Now try using it with a keyboard.
Open the dialog. Press Tab. Where does focus go? If it goes behind the dialog to the page content, the dialog...
🛡️ VERIFIED CYBER INTELLIGENCE ID: #3302265