This specification indicates whether a section is normative or non-normative (informative) and the classification applies to the entire section. A statement "This section is normative" or "This section is non-normative" applies to all sub-sections of that section.
Normative sections provide requirements that user agents must follow for an implementation to conform to this specification. The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, RECOMMENDED, MAY, and OPTIONAL in this document are to be interpreted as described in Keywords for use in RFCs to indicate requirement levels [[!RFC2119]]. RFC-2119 keywords are formatted in uppercase and contained in a strong element with class="rfc2119". When the keywords shown above are used, but do not share this format, they do not convey formal information in the RFC 2119 sense, and are merely explanatory, i.e., informative. As much as possible, such usages are avoided in this specification.
Normative sections provide requirements that authors, user agents and assistive technologies MUST follow for an implementation to conform to this specification.