Prune instructs the parser to remove any elements within the extracted article block that look superfluous. This can result in false positives, so we tend to disable it when we've gone to the trouble of creating site-specific extraction rules.
Tidy determines if the source HTML should be cleaned up first with HTML Tidy - https://github.com/htacg/tidy-html5. If you're parsing the source HTML with an HTML 5 parser, as we are now, it shouldn't be necessary any more (I think we actually ignore it now). We used it more before when we relied on libxml parsing, which often trips up on modern HTML.
