Angular is built from the Web Components standard, a set of technologies whom one is the Shadow DOM.
The Shadow DOM allows to define a local behavior of a webpage in the DOM, without interfering wit other parts of your application: events, HTML elements, CSS style, etc.
Angular provides three strategies to achieve Shadow DOM:
It is the default strategy that Angular uses, it simulates the behavior of the Shadow DOM, but it is able to work on any browser, because the Shadow DOM is a recent feature, hence not supported by all browsers.
This strategy supports the specifications of the Shadow DOM. It is useless, as default mode covers it.
It is a feature to disable the component encapsulation. You risk to have collision between your components.
The most common used strategy is Emulated.