PowerApps Accessibility Standards and Guidelines
https://powerapps.microsoft.com/en-us/blog/powerapps-canvas-app-accessibility-guidelines/
Power Apps portals conforms to the WCAG 2.1 accessibility standard
https://docs.microsoft.com/en-us/powerapps/maker/portals/admin/accessibility
ARIA Authoring Practices Guide
WCAG 2.0 and 2.1:
http://usability.com.au/2008/09/accessible-forms-using-wcag-2-0/#comment-87243
H90: Indicating required form controls using label or legend | WAI | W3C
Useful links:
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/accessible-apps
https://www.456bereastreet.com/archive/200505/voiceover_and_safari_screen_reading_on_the_mac/
https://support.google.com/accessibility/android/answer/6283677