Webflow's Rich Text Block ( RTB ) is one of the most important content elements in Webflow, acting as the core of many pages.

It supports basic text formatting;

Bold and italics
Superscript and subscript
Good links support
> Page links, CMS page links, URL links, email and phone
> Open in new tab support
> SEO _rel attribute support

It also has support for specific sub-element types ( RTEs ) within your RTB;

Paragraphs
Headings
Lists, both ordered an unordered - but 1 level only
Blockquotes
Images, including some limited sizing and floating options
Video elements and multimedia elements
Custom HTML embeds, for unsupported HTML structures, CSS, Javascript, etc.
?? Code blocks ( new ) for adding formatted code snippets on your site - may not be part of RTBs.

17-Dec-2023 a new Code Block element was added, however it does not yet appear to be available within RTBs. At a minimum, it does not show in Editor mode yet.