The browser should do it. Websites often do this intentionally in order to trick users into clicking ads. The only way to stop these abuses is for the browser itself to stop enabling them.
Unfortunately Google would make the Gmail team very grumpy if they did this - Gmail's by far the worst offender in my experience. Every single time I open my email I have to remind myself to wait for a moment before clicking on my newest emails, because most of the time there's going to be a couple ad "emails" popping in and shuffling everything else down a few seconds after all the real emails have loaded.
Google's grumpiness is an excellent measure for how much we need a feature. The grumpier, the better. Other browsers should do it. Firefox, Safari, Brave, everyone else.
The browser should do it. Websites often do this intentionally in order to trick users into clicking ads. The only way to stop these abuses is for the browser itself to stop enabling them.