A web page isn’t locked in stone just because it has rendered visually. Media assets, like images, can come in and cause the layout to shift based on their size, which typically isn’t known in fluid layouts until they do render. Or fonts can load and reflow layout. Or XHRs can bring in more content to be placed onto the page. We’re always doing what we can to prevent the layout from shifting around — that’s what I mean by layout jank. It’s awkward and nobody likes it. At best, it causes you to lose your place while reading; at worst, it can mean clicking on something you really didn’t mean to.
While I was trying to wrap my head around the new Layout Instability API and chatting it out with friends, Eric Portis said something characteristically smart. Basically, layout jank is a problem and it’s being fought on multiple fronts: