This is one of the worst UI trends in the last decade. Mock content gives devs permission to ship slow APIs, and they can even look janky when your API is too fast. It feels like watching a half hour of trailers before the movie starts.
Well, they definitely don't solve the problem for the user, as the grey boxes inevitably aren't the same shape as the content that eventually appears, and the jumping around is still there. Text fields are often variable length and can reflow. Buttons are also frequently dynamic, and all sorts of widgets may not just line up. Sometimes, incomprehensive design changes cause further disalignment between the skeleton and the actual content.
It's sad and funny that the YouTube app seems to have just recently jumped on this bandwagon (either that, or the situation worsened so much that I started noticing how bad it is).
And I can't even count the number of web sites (from companies with plenty of engineering budget!) that render the gray placeholder UI, but when the real data comes in, it doesn't even remotely line up with the placeholder UI (which is the whole point). I totally get how it happens: the placeholder UI and real data UI isn't sharing code, and just relies on dumb luck that the next person who changes the real data UI also changes the placeholder UI to match it.
This is one of the worst UI trends in the last decade. Mock content gives devs permission to ship slow APIs, and they can even look janky when your API is too fast. It feels like watching a half hour of trailers before the movie starts.