Different types of observers supported by modern browsers?

Before diving deeper into the observers family supported by modern browsers, lets try to understand what is observer in general?

What is Observer?
Observer is a programme which watches or notices something. Observers can be helpful to watch certain activities happening in the browser and respond accordingly. Observers are similar to dogs, watching certain activities and alerting us if something unusual happens. And its our responsibility to take an action once we are alerted by dogs for certain activities.