This word, or the terms "REQUIRED" or "SHALL", mean that the definition is an absolute requirement of the specification.Must Not
This phrase, or the phrase "SHALL NOT", means that the definition is an absolute prohibition of the specification.Should
This word, or the adjective "RECOMMENDED", means that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.Should Not
This phrase, or the phrase "NOT RECOMMENDED" means that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.May
This word, or the adjective "OPTIONAL", means that an item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because the vendor feels that it enhances the product while another vendor may omit the same item. An implementation which does not include a particular option MUST be prepared to interoperate with another implementation which does include the option, though perhaps with reduced functionality. In the same vein an implementation which does include a particular option MUST be prepared to interoperate with another implementation which does not include the option (except, of course, for the feature the option provides.)
-- Definitions from RFC 2119 - Key words for use in RFCs to Indicate Requirement Levels (http://www.rfc-editor.org/rfc/rfc2119.txt)Deprecated
A deprecated element or attribute is one that has been outdated by newer constructs. Deprecated elements are defined in the reference manual in appropriate locations, but are clearly marked as deprecated. Deprecated elements may become obsolete in future versions of HTML.
User agents should continue to support deprecated elements for reasons of backward compatibility.
Definitions of elements and attributes clearly indicate which are deprecated.
This specification includes examples that illustrate how to avoid using deprecated elements. In most cases these depend on user agent support for style sheets. In general, authors should use style sheets to achieve stylistic and formatting effects rather than HTML presentational attributes. HTML presentational attributes have been deprecated when style sheet alternatives exist (see, for example, [CSS1]).Obsolete
An obsolete element or attribute is one for which there is no guarantee of support by a user agent. Obsolete elements are no longer defined in the specification, but are listed for historical purposes in the changes section of the reference manual.
-- Definitions from HTML 4.0 Reference Specification Section 4.1 - Definitions (http://www.w3.org/TR/html4/conform.html)