3 Conformance: Requirements and Recommendations   3.1
Definitions  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 RFC 2119 (see [RFC2119]). However, for readability,
these words do not appear in all uppercase letters in this
specification.  At times, this specification recommends 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.  The fact that a feature is
marked as deprecated (namely the 'aural' keyword) or going to be
deprecated in CSS3 (namely the system colors) also has no
influence on conformance. (For example, 'aural' is marked as
non-normative, so UAs do not need to support it; the system
colors are normative, so UAs must support them