Press Edit, click where you want the words, and type. The toolbar above the text sets what kind of line it is: a paragraph, a heading, a list, a quote.
What you can use
Bold (⌘B) and italic (⌘I). 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.
Why it stops there
Fewer choices, fewer ways to make a page worse. Pages read well without anyone having to be a designer.
Pasting is safe for the same reason. Paste from a word processor, a browser or an email. Formatting that has no place here is dropped, and the words arrive clean.
What you cannot do
Colours, fonts, custom HTML, scripts, or putting things side by side. If you want a site that looks unlike everyone else's, this is the wrong tool.