What is the detailed process of generating Render tree?

Question

I konw that in the process of generating Render tree, Dom tree is read from the root node, and for each node in Dom tree, the matching CSSOM rule is found. Since CSS selectors is matched from right to left, if it means that for each node in DOM tree, the browser will start searching CSSOM tree from all its leaf nodes instead of the root node? And if my opinion is correct, how can browser efficently get all the leaf nodes of CSSOM tree?