The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in such documents are to be interpreted as described in RFC 2119.
When using RFC 2119 terms, consider re-using the mark-up pattern from the W3C Manual of Style (in particular
class="RFC2119" - note capitalization), and perhaps their suggested style rule as well.
In addition, see Edward O’Connor POSH suggestion for marking up the commonly used initial paragraph declaring RFC 2119 terms in documents.
Some have suggested considering a new microformat for marking up use of RFC 2119 terms.
It appears there is no need for such a new microformat however. Following W3C's established practice, and Edward O'Connor's POSH suggestion for the initial paragraph, appears to be sufficient.