For what I've seen working on MDL-30899 moodle displays YUI modal windows and overlays in diferent ways, most of them uses the main notifications YUI classes (lib/yui/notification) but there are others (block community comments, block community images, assign new enrol, enrolment cohort, enrolment manual...) that are implemented differently, this leads to different UI (for example enrol-cohort and enrol-manual) duplicate code (close buttons management, loading overlays...) and accessibility problems that are hard to manage in a centralized way.

I restricted MDL-30899 actions to solve the main accessibility problems, but I'm opening a new issue to discuss if more work should be done to centralize the management of modal windows / panels / overlays and if more overlays should become modal windows for all the advantages they have over regular overlays in user experience and accessibility terms.