Wrong containing block identified for elements with an animation

1. Visit https://scroll-driven-animations.style/
2. Inspect one of the buttons in the cards
3. Execute the following:
```js
const { getContainingBlock } = await import("https://esm.sh/get-containing-block@1.0.3");
$0.style.position = 'absolute';
$0.style.top = 0;
getContainingBlock($0);
```
4. Check the reported contents

The reported containing block is `null` (ICB) whereas it should be the wrapping `.card` here is the CB.

Reproduce the report on https://scroll-driven-animations.style/ using the provided import and steps, then start at the getContainingBlock entry point. Check the containing-block detection for an absolutely positioned card button and make the result identify the wrapping .card rather than null; verify the reported contents match the expected containing block.

Written by the indexing model from the issue text.