4. Conformance:requirements and recommendations

In this section, we begin the specification of HTML 4, starting with thecontract 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 documentare to be interpreted as described in [RFC2119]. However, forreadability, these words do not appear in all uppercase letters in thisspecification.

At times, the authors of this specification recommend good practice forauthors and user agents. These recommendations are not normative andconformance with this specification does not depend on their realization. Theserecommendations contain the expression "We recommend ...", "This specificationrecommends ...", or some similar wording.

4.1 Definitions

HTML document

An HTML document is an SGML document that meets the constraints of thisspecification.

Author

An author is a person or program that writes or generates HTML documents.An authoring tool is a special case ofan author, namely, it's a program that generates HTML.

We recommend that authors write documents that conform to the strict DTD rather than the other DTDs defined by thisspecification. Please see the section on version information for details about theDTDs defined in HTML 4.

User

A user is a person who interacts with a user agent to view, hear, orotherwise use a rendered HTML document.

HTML user agent

An HTML user agent is any device that interprets HTML documents. Useragents include visual browsers (text-only and graphical), non-visual browsers(audio, Braille), search robots, proxies, etc.

A conforming user agent for HTML4 is one that observes the mandatory conditions ("must") set forth in thisspecification, including the following points:

• A user agent should avoid imposing arbitrary length limits on attributevalue literals (see the section on capacities in the SGML Declaration). For introductory information onSGML attributes, please consult the section on attribute definitions.
• A user agent must ensure that rendering is unchanged by the presence orabsence of start tags and end tags when the HTML DTD indicates that these areoptional. See the section on element definitions for introductory information on SGML elements.
• For reasons of backwards compatibility, we recommend that toolsinterpreting HTML 4 continue to support HTML 3.2 (see [HTML32]) and HTML2.0 (see [RFC1866]).

Error conditions

This specification does not define how conforming user agents handlegeneral error conditions,including how user agents behave when they encounter elements, attributes,attribute values, or entities not specified in this document.

However, for recommended error handling behavior, please consult the notes on invaliddocuments.

Deprecated

A deprecated element or attribute is one that has been outdated by newerconstructs. Deprecated elements are defined in the reference manual inappropriate locations, but are clearly marked as deprecated. Deprecatedelements may become obsolete in future versions of HTML.

User agents should continue to support deprecatedelements for reasons of backward compatibility.

Definitions of elements and attributes clearly indicate which aredeprecated.

This specification includes examples that illustrate how to avoid usingdeprecated elements. In most cases these depend on user agent support for stylesheets. In general, authors should use style sheets to achieve stylistic andformatting effects rather than HTML presentational attributes. HTMLpresentational attributes have been deprecated when style sheet alternativesexist (see, for example, [CSS1]).

Obsolete

An obsolete element or attribute is one for which there is no guarantee ofsupport by a user agent. Obsolete elements are no longerdefined in the specification, but are listed for historical purposes in the changes section of the reference manual.

4.2 SGML

HTML 4 is an SGML application conforming to InternationalStandard ISO 8879 -- Standard Generalized Markup Language SGML(defined in [ISO8879]).

Examples in the textconform to the strict document type definitionunless the example in question refers to elements or attributes only defined bythe transitional document type definition orframeset document type definition. For thesake of brevity, most of the examples in this specification do not begin withthe document type declarationthat is mandatory at the beginning of each HTML document.

DTD fragmentsin element definitions come from the strict documenttype definition except for the elements related to frames.

Please consult the section on HTMLversion information for details about when to use the strict, transitional,or frameset DTD.

Comments appearing in the HTML 4 DTD have no normative value; they are informative only.

User agents must not render SGML processing instructions (e.g., <?fullvolume>) or comments. For more information about this and otherSGML features that may be legal in HTML but aren't widely supported by HTMLuser agents, please consult the section on SGML features with limited support.

4.3 The text/html content type

HTML documents are sent over the Internet as a sequence of bytes accompaniedby encoding information (described in the section on character encodings). The structure of thetransmission, termed a message entity, is defined by [RFC2045] and [RFC2616]. A message entity with a content type of "text/html" represents anHTML document.

The content type for HTML documents is defined asfollows:

Content type name:

text

Content subtype name:

html

Required parameters:

none

Optional parameters:

charset

Encoding considerations:

any encoding is allowed

Security considerations:

See the notes onsecurity

The optional parameter "charset" refers to the character encoding used to represent theHTML document as a sequence of bytes. Legal values for this parameter aredefined in the section on characterencodings. Although this parameter is optional, we recommend that it alwaysbe present.