From the html 4.01
spec:
In this section, we begin the specification of HTML 4, starting with the
contract between authors, documents, users, and user agents.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in [RFC2119]. However, for readability, these words do not
appear in all uppercase letters in this specification.
At times, the authors of this specification recommend good practice for
authors and user agents. These recommendations are not normative and conformance
with this specification does not depend on their realization. These
recommendations contain the expression "We recommend ...", "This specification
recommends ...", or some similar wording.