What prevents you from wrapping the whole page contents in another container to select?

This is indeed a common workaround; but I dislike adding another <div class="wrapper"> in wikitext of all pages where the templateStyles is included, whereas there is already a <div class="mw-parser-output"> which could be used for this purpose.

I would like > selector to be parsed into .mw-parser-output > selector.

Definitely not; it's invalid CSS, which makes it confusing and breaks syntax highlighting in the editor.

We could skip prefixing of selectors which start with .mw-parser-output > - pretty sure that came up in another task already.

In T340930#8984262, @Tgr wrote:

We could skip prefixing of selectors which start with .mw-parser-output > - pretty sure that came up in another task already.

T271588. Could make it so that only some combinators are supported like > as you suggest, indeed (but would need to dodge supporting naked .mw-parser-output and e.g. .mw-parser-output +, which doesn't seem difficult). I don't really understand what need is motivating Pols here so I can't say whether this task is more motivational.

That was the one, thanks. Styling .mw-parser-output can be somewhat security-sensitive; styling its children isn't. It would complicate the logic of a security-sensitive operation (selector prefixing) though, so I'm not sure it's worth it. I think Parsoid will at some point introduce <section> tags wrapping most content, so this problem will go away eventually, anyway.

A kind of subset of T271588, indeed.

The use cases are on Meta-Wiki, where project pages often define a specific layout (example). Having <section> tags would probably workaround this issue, indeed.

On that particular page, you could use the sibiling combinator with .mw-heading2 ~ p {}?

In T340930#8984399, @Izno wrote:

On that particular page, you could use the sibiling combinator with .mw-heading2 ~ p {}?

Indeed, .mw-heading2 ~ ul, .mw-heading2 ol { margin-left: 6ex; } is also a nice workaround as soon as there is at least one h2; thank you.