system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do mobile websites load, and load again, and change their spacing again? Oops, and there's an ad. Better jump the screen around a bit.
gpt
Can't speak for other developers, but in order to load some of our ads we need third party code which needs to be downloaded too. These scripts take longer to load than other smaller assets, like text and images. So the content will jump around a bit to account for the ads that have loaded in the meantime.

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The jumping around could be resolved by using placeholders of the exact same size as the ads, but we don't always know which banner size we will load ahead of time.