But the underlying principle is still filtering out the undesired content instead of selecting the desired content.

Surely the principle the parent suggested was to initially filter out all of the content, and then selecting the desired content to load.

But isn't "filtering out all of the content" just another way of saying, "not loading any content" - until explicitly requested?

Perhaps I misunderstood. But let's say I do run uMatrix in deny more, I get the site HTML as the Tim Berners-Lee intended. Yet that already may contain some kind of advertising material, so to view only the actual content I'd need to have e.g. an XPath do it. So I would have a database of sites and XPath (or other ways) describing how to find the content, instead of describing how to get rid of non-content.

Can I tell uMatrix to only show a certain XPath from the page? If so, I agree it is a content selector, albeit pretty impractical :).

uMatrix is the absolute best, I can't imagine browsing without it. So the unmaintaned part is very sad and troubling.

I do install uBlock Origin for all non-technical family members, but uMatrix is so much more capable.