CSSOM, together with DOM, is necessary to build the render tree, which in turn is used by the browser to lay out and paint the web page.

Like HTML and DOM, when CSS files are loaded, they need to be parsed and transformed into a tree - this time, CSSOM. It describes all the CSS selectors on the page, their hierarchy, and their properties.

CSSOM differs from DOM in that it can't be constructed incrementally, as CSS rules can overwrite each other due to specificity. This is why CSS blocks rendering, as until all CSS is parsed and CSSOM is built, the browser can't know where and how to place each element on the screen.

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