One common trick web authors do because they think it’s neat is make elements of a webpage fixed so as the user scrolls around part of the webpage stays still. “This is so important the viewer must always see it!”, they think.
Sometimes it’s done with good intents; a rich web app may decide a toolbar should always be visible, for instance. Usually it’s done with scummy intents; “If I make the viewer see SHARE ME! buttons wherever they scroll, eventually they’ll give up and do it!”.
Either way, read on for a simple visual guide as to why this must stop.
(I don’t mean to pick on these sites especially, and I’m sorry. Your just one of many that do this, and you were a handy example. Nothing personal.)
Now, do you all remember that browsing on mobile devices is on the rise? Good. What do many mobile devices have in common? A small screen. What do people do on small screens? They zoom in to read the text. What happens with your clever floating menu then?
Congratulations. Your website is now completely unreadable. Here are some more examples:
In summary: use only when appropriate (clue: almost never), and test carefully.