Two that spring to mind (there are possibly many more).
One is to just put a <style> section on the page head with the page’s unique css rules.
Another is to have an ID on the <body> then use that in selectors in the global css to target only elements on those pages.
I’m not sure what any of that paragraph means 
. All I know is there is no reason I can think of with modern css, to be using iframes for layout purposes.