Задача решена? https://codepen.io/istevedogs/pen/JjzvZaw

Внимательно прочитай условие)
Должно быть наоборот, кнопка на которую наводишь остается, а другие исчезают

Вот так? - https://codepen.io/istevedogs/pen/JjzvZaw

span:has(~ span:ishover)) {
opacity: 0;
}

span:hover ~ span {
opacity: 0;
}

I want to participate but is hard to understand russian))

The css to do this would be the use of :has and ~

:has To select the previous siblings:

~ To select the next siblings:

https://codepen.io/ofuwdwfe-the-animator/pen/wvOjxrP

I want to participate but is hard to understand russian))

https://codepen.io/ofuwdwfe-the-animator/pen/wvOjxrP

Task:
There are 2 buttons, when we hover the cursor over one of them, the second disappears.
The task needs to be solved only using HTML and CSS
without using javascript

Yes, this is a good solution.

Try to solve this without using the pseudo-classes :is() and :has()
You can solve this using only one pseudo-class :hover. Then it will be more cross-browser

*, button:hover {
opacity: 0;
}