Define and Explain the DOM
DOM stands for Document Object Model, and is a programming interface for web documents. It represents the page as nodes and objects, and allows programs to change the structure/style/content of a page. DOM is independent of any programming language.
DOM Specifications
The most recent specification for DOM is level 4. Levels represent different versions of the specification for how the DOM works. DOM level 1 defines the core elements of DOM, level 2 extends those elements and adds events. Level 3 add more elements and events.
DOM Implementation in Browsers
Due to the existence of a multitude of browsers (Chrome, Firefox, Safari, Edge, Opera, Android equivalents, etc.) each browser can have varying support for each DOM specification. Online resources are available to review which browser supports which DOM specification.
Summary of the Documentation
The DOM allows languages like JavaScript to interact with a web document. There are multiple specifications of the Document Object Model, and knowing which browser supports each will help you design websites that can be viewed without issue by your target audience.