When I was first styling Resilient Web Design, I made heavy use of
vh units. The vertical spacing between elements—headings, paragraphs, images—was all proportional to the overall viewport height. It looked great!
Then I tested it on real devices.
Here’s the problem: when a page loads up in a mobile browser—like, say, Chrome on an Android device—the URL bar is at the top of the screen. The height of that piece of the browser interface isn’t taken into account for the viewport height. That makes sense: the viewport height is the amount of screen real estate available for the content. The content doesn’t extend into the URL bar, therefore the height of the URL bar shouldn’t be part of the viewport height.
But then if you start scrolling down, the URL bar scrolls away off the top of the screen. So now it’s behaving as though it is part of the content rather than part of the browser interface. At this point, the value of the viewport height changes: now it’s the previous value plus the height of the URL bar that was previously there but which has now disappeared.
In my initial implementation of Resilient Web Design, the one where I was styling almost everything with
vh, the site was unusable. Every time you started scrolling, things would jump around. I had to go back to the drawing board and remove almost all instances of
vh from the styles.