CORE AAM says:
1. When exposing as a platform API Boolean state:
  A. For values of "" (empty string), "undefined" or no attribute present:
     - Follow the guidance in the Default values for missing required attributes table below, if applicable.
     - Otherwise, treat as false.
  B. Treat any other value as true.
2. Otherwise, ignore the value and treat the property as not present.
 

CORE AAM says:
If a WAI-ARIA property contains an unknown or disallowed value, the user agent should expose to platform accessibility APIs as follows:
1. When exposing as a platform API Boolean state:
  A. For values of "" (empty string), "undefined" or no attribute present:
     - Follow the guidance in the Default values for missing required attributes table below, if applicable.
     - Otherwise, treat as false.
  B. Treat any other value as true.
2. Otherwise, ignore the value and treat the property as not present.
 

CORE AAM says:
If a WAI-ARIA property contains an unknown or disallowed value, the user agent should expose to platform accessibility APIs as follows:
1. When exposing as an object attribute, expose the unknown value — do not vet it against possible values.
2. When exposing as a platform API Boolean state:
  A. For values of "" (empty string), "undefined" or no attribute present:
     - Follow the guidance in the Default values for missing required attributes table below, if applicable.
     - Otherwise, treat as false.
  B. Treat any other value as true.
2. Otherwise, ignore the value and treat the property as not present.