The HTML for a <details> element is generally something like:
<details>
<summary>Spoiler alert!</summary>
<p>Rosebud was a sled.</p>
<p>It's true!</p>
</details>Code language: HTML, XML (xml)
See how I put two <p> elements in there? That’s totally fine. Everything that isn’t the <summary> is visually hidden until the <details> is open, either via the open attribute or the summary is clicked/tapped.
So if you’re trying to select “all the content”, you’re either forced to use a wrapper or use an awkward selector like details > *:not(summary). The newly-specced ::details-content looks like a nice fix for this. (hat tip).