Sometimes it would be handy to have a possibility to annotate/highlight some rules, behaviors or exceptions so developers can see them while developing a feature. For example, when the behavior of an element depends on the settings made in a different part of the application (that is not present on the screen).
I know there is a possibility to comment, but I think about something apart - that it wouldn’t be impossible to omit in the inspect mode, doesn’t cover much of the design and is clearly different from comments.