Accessibility
aria-hidden. The required label renders as visually hidden text after the children, so it joins the host's accessible name: a tab reads "Learning, New activity", and an icon-only button reads "Notifications, Needs attention". Put the severity wording in the label.
Colour only reinforces. The dot's presence, the hidden label and the surface the marker points to carry the meaning, so nothing depends on telling amber and red apart. The marker has no motion, so there is nothing to reduce. In forced-colors mode the dot keeps a fill and a border, and in right-to-left languages the corner dot moves to the left corner.