Hiding, showing, and loading content across viewports

One of the commonly touted maxims regarding responsive web design is: if you don't have something on the screen at smaller viewports, you shouldn't have it there at larger ones either.

This means users should be able to accomplish all the same goals (buy a product, read an article, accomplish an interface task) at every viewport size. This is common sense. After all, as users ourselves, we've all felt the frustration of going to a website to accomplish a goal and being unable to, simply because we're using a smaller screen.

It also means that as screen real estate is more plentiful, we shouldn't feel compelled to add extra things just to fill the space (widgets, adverts, or links for example). If the user could live without those extras at smaller screen sizes, they'll manage just fine at bigger ones. Displaying extra content at larger viewport sizes also means that either the content was there at smaller viewports and was merely hidden...