The World Wide Web Consortium standards for representation of documents (for instance, Html Documents with Cascading Style Sheets and Extensible Markup Language documents) as an object model. Three levels of the specification exist, where each is a superset of the last, and levels 2 and 3 are separated into modules. See www.w3.org 
for details. Level 1 and most of level 2 are supported by major web browsers for Javascript (the primary audience).