The CSS Object Model (CSSOM) is a map of all CSS selectors and relevant properties for each selector in the form of a tree, with a root node, sibling, descendant, child, and other relationship. The CSSOM is very similar to the Document Object Model (DOM). Both of them are part of the critical rendering path which is a series of steps that must happen to properly render a website.
The CSSOM, together with the DOM, to build the render tree, which is in turn used by the browser to layout and paint the web page.
CSSOM API
The CSS Object Model is also a set of APIs allowing the manipulation of CSS from JavaScript. It is much like the DOM, but for the CSS rather than the HTML. It allows users to read and modify the CSS style dynamically.