Complex sentence: The sequence in which the various elements making up a Web page is almost never strictly top-to-bottom, the programming that fills in the missing pieces must do a certain amount of guesswork to determine how to best display partial content.
Split into simpler sentences: The sequence in which the various elements of a Web page render is almost never strictly top-to-bottom. The programming that fills in the missing pieces must do a certain amount of guesswork to determine how to best display partial content.