When a page contains several input fields that have been marked as required, and the user submits the form, Edge will add a red outline on all fields, and show an error bubble next to the first field with an error (like most browsers).

It's possible for a website to use the :invalid pseudo selector to add their own styles. But if the field is shown with default styles, then it should be the case that "color is not used as the only visual means of conveying information", which is the case when the error bubble has disappeared.

This means the default browser styles fails WCAG Guideline 1.1.4:

https://www.w3.org/TR/WCAG20/#visual-audio-contrast-without-color

Maybe an exclamation mark could be shown to the right of the input field (or as a background image)?