More and more web pages have broken the page down key. Fixed-height headers and footers on top of web content don’t bother me, but the article underneath them never pages correctly. Because it’s paging the full window height and the header/footer has hidden part of the document, so you never see it.
This used to only be a problem on shitty websites that had all sorts of bad layout. But now even high design websites like the In Focus photojournalism gallery have broken the paging behavior with a header. Medium was a holdout for a long time, it even had a fancy thing where the header hid itself when you scrolled down a page. It still does that but now there’s a fixed 20px footer that obscures text that disappears when you page down.