What you can use
Bold, italic, two levels of heading, bulleted and numbered lists, links, quotes, an aside for a short note, a horizontal rule, inline code and code blocks. Emoji work anywhere text works.
That is the whole list.
Why it stops there
Every option is a decision, and every decision is a chance to make your page worse. A writing surface with eleven choices produces pages that read well without anybody having to be a designer.
It also means pasting from anywhere is safe. Paste from a word processor, a browser, an email: the formatting that has nowhere to live is simply dropped, and the words arrive clean.
What you cannot do
No colours, no fonts, no custom HTML, no scripts, no arranging things side by side. If you want a page that looks unlike everybody else's, this is the wrong tool and that is fine.