This[1] bookmarklet strips everything that isn't preformatted plain text from a page. It seems to work on both gametabs[2] and ultimate-guitar[3] and probably others that don't try to get fancy with their typography. Hopefully it is of some use.
[1](javascript:(function(){const tabs = document.querySelectorAll('pre'); document.head.remove(); document.body.remove(); document.body = document.createElement('body'); for(const tab of tabs){ document.body.appendChild(tab);};})();)