As for why the parent render tree invalidation is necessary, consider <style> main:only-child * { /* ... */ } </style> <body> <main>lot

As for why the parent render tree invalidation is necessary, consider <style> main:only-child * { /* ... */ } </style> <body> <main>lots of stuff </main> </body> When second node is appended to <body> our CSS selector no longer matches so the rule stops being applied and content of the <main> is no longer styled. When there is (next) sibling wrapper present, anything what happens inside it cannot affect the <main> in our example: in CSS there simply is currently no way to target element according it's next sibling inner structure. (There is one for previous sibling: `prev:empty + next {}`: when prev stops being empty, rule stops to match.)