Why is timestables.com misbehaving on iPads outside of Safari?

[gutley]

Near the bottom of www.timestables.com are some math games for younger students.

On all Webkit-based iOS browsers, except Safari, these games will not allow any interaction if they were loaded via "Request Desktop Site".

E.g https://www.timestables.com/my-smart-horse.html?play will display just fine, but attempting to press any of the in-game buttons does nothing.

Switching to mobile version of the page looks identical, but works as expected.

On iPad, Firefox defaults (correctly) to using the Desktop versions of websites, as does Safari, but these games work OK in Safari regardless of whether requesting mobile or Desktop site.

Does anyone have any insights into why only Safari works properly with this site?