The design of CSS is such that pretty much any element in the document can affect any other element. Therefore determining what is off-screen generally requires

The design of CSS is such that pretty much any element in the document can affect any other element. Therefore determining what is off-screen generally requires processing the entire document. The manual property changes the behavior of CSS so that elements in a subtree cannot affect elements outside the subtree, allowing the browser to quickly discard whole subtrees instead.