I'd argue the best place for referencing a style sheet is at the page level (or its template), regardless of CMS approach. Consider approaching the "embed an internal style sheet" requirement with page metadata or as an external link for components. I describe other [RTF approaches in this post][1].

If the HTML is maintained elsewhere and is simply copy/pasted into components, **consider a text box instead of RTF**. I wouldn't say it's ideal, but it'll save headaches as you work through three or four layers when forcing RTF to act like HTML:

 1. Allowed nodes
 1. Per-field XSLT filter
 1. Stored XML and any differences in APIs accessing it
 1. Template logic

**This being said, if this really needs to be *in* the RTF, look at [`TcmXhtml.Config`](https://stackoverflow.com/a/9618578/395495) for adjusting the allowed nodes as well as the RTF's XSLT filter.**

The rich text format area is *not* an *HTML* editor, there will be pain when forcing it to act like one and again when changing styles, system, and/or approach. :-)

  [1]: http://www.tridiondeveloper.com/rich-text-format-area-css-classes-vs-custom-xml-nodes