I get your point, but the RSS format (and Atom, and JSON feed..) and the feeds are also partially to blame: there is just no common HTML subset specified for RSS. Some feeds are formatting their posts specifically for RSS. But they are in the minority. Most are just dumping whatever HTML they put on the website. With CMS-specific CSS classes and Tailwind CSS, without declaring them anywhere. Or even requiring JavaScript.
RSS Readers have no chance but remove most styles from the HTML and apply their own, hoping it renders ok.