The CSS Object Model (CSSOM), part of the DOM, exposes specific interfaces allowing manipulation of a wide amount of information regarding CSS. Initially defined in the DOM Level 2 Style recommendation, these interfaces forms now a specification, CSS Object Model (CSSOM) which aims at superseding it.
The Web platform provides several ways to get notified of DOM events. Two common styles are: the generalized addEventListener() and a set of specific on-event handlers. This page focuses on the details of how the latter work.