1. You can use CSS, but expect that most readers will completely ignore it. Even widespread ones, like Feedly. Those that do support it will only offer a small subset, like text styles. No layout styles (positioning or flex-layouts), often not even colors. If you want to try the capabilities of your reader, this is a test feed: https://test-rss.downread.com/styles.rss.xml

2. Don't just prefer inline CSS, use it, because any sane RSS reader will filter <style>.

3. Yes, semantic elements will generally work. And not only those listed: also things like <table>, <figure>, and sometimes <aside>.

4. Agree about JavaScript. Don't use it.

5. Decent readers will support <video> and <audio>. But <iframes> are even better, as long as you use common providers, especially YouTube. Embeds from X, Instagram, and Facebook are also relatively widely supported.

6. Agree about forms. At best, they will be used as rendering hints, but forms won't work.