The answer
Because they can force the browser to recalculate styles, rebuild render information, and rerun layout or paint work.
Browser DOM, CSSOM, and Render Tree Explained
Why do DOM and CSS changes affect performance?
More FAQs in Browser DOM, CSSOM, and Render Tree Explained
The DOM is the document object model built from HTML. It represents page structure as nodes that scripts can read and change.
The CSSOM is the browser's model of parsed CSS rules and computed styles used to determine how the page should look.
The render tree combines DOM and CSSOM information into the visible objects the browser can lay out and paint.
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