Techopedia explains
Document Object Model (DOM)
A Web standard defined by the World Wide Web Consortium (W3C), DOM is supported by most browsers. The DOM allows a web-developer to access the document through a common set of objects, properties, methods and events and allows webpage's content to be altered dynamically with a scripting language.
Scripting languages have several variations in syntax, as well as vendor-added variations, which are common issues in Web application implementations. The W3C has provided the DOM standard as a generic set of objects, properties and methods that are available to all scripting languages. However, no browser is 100 percent DOM-compliant. Thus, there is no assurance that W3C’s DOM standard is available in every browser