RFC 1866
Hypertext Markup Language - 2.0
Network Working Group T. Berners-Lee Request for Comments: 1866 MIT/W3C Category: Standards Track D. Connolly November 1995 Hypertext Markup Language - 2.0 Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Abstract The Hypertext Markup Language (HTML) is a simple markup language used to create hypertext documents that are platform independent. HTML documents are SGML documents with generic semantics that are appropriate for representing information from a wide range of domains. HTML markup can represent hypertext news, mail, documentation, and hypermedia; menus of options; database query results; simple structured documents with in-lined graphics; and hypertext views of existing bodies of information. HTML has been in use by the World Wide Web (WWW) global information initiative since 1990. This specification roughly corresponds to the capabilities of HTML in common use prior to June 1994. HTML is an application of ISO Standard 8879:1986 Information Processing Text and Office Systems; Standard Generalized Markup Language (SGML). The "text/html" Internet Media Type (RFC 1590) and MIME Content Type (RFC 1521) is defined by this specification. Table of Contents 1. Introduction ........................................... 2 1.1 Scope .................................................. 3 1.2 Conformance ............................................ 3 2. Terms .................................................. 6 3. HTML as an Application of SGML .........................10 3.1 SGML Documents .........................................10 3.2 HTML Lexical Syntax ................................... 12 3.3 HTML Public Text Identifiers .......................... 17 3.4 Example HTML Document ................................. 17 4. HTML as an Internet Media Type ........................ 18
4.1 text/html media type .................................. 18
4.2 HTML Document Representation .......................... 19
5. Document Structure .................................... 20
5.1 Document Element: HTML ................................ 21
5.2 Head: HEAD ............................................ 21
5.3 Body: BODY ............................................ 24
5.4 Headings: H1 ... H6 ................................... 24
5.5 Block Structuring Elements ............................ 25
5.6 List Elements ......................................... 28
5.7 Phrase Markup ......................................... 30
5.8 Line Break: BR ........................................ 34
5.9 Horizontal Rule: HR ................................... 34
5.10 Image: IMG ............................................ 34
6. Characters, Words, and Paragraphs ..................... 35
6.1 The HTML Document Character Set ....................... 36
7. Hyperlinks ............................................ 36
7.1 Accessing Resources ................................... 37
7.2 Activation of Hyperlinks .............................. 38
7.3 Simultaneous Presentation of Image Resources .......... 38
7.4 Fragment Identifiers .................................. 38
7.5 Queries and Indexes ................................... 39
7.6 Image Maps ............................................ 39
8. Forms ................................................. 40
8.1 Form Elements ......................................... 40
8.2 Form Submission ....................................... 45
9. HTML Public Text ...................................... 49
9.1 HTML DTD .............................................. 49
9.2 Strict HTML DTD ....................................... 61
9.3 Level 1 HTML DTD ...................................... 62
9.4 Strict Level 1 HTML DTD ............................... 63
9.5 SGML Declaration for HTML ............................. 64
9.6 Sample SGML Open Entity Catalog for HTML .............. 65
9.7 Character Entity Sets ................................. 66
10. Security Considerations ............................... 69
11. References ............................................ 69
12. Acknowledgments ....................................... 71
12.1 Authors' Addresses .................................... 71
13. The HTML Coded Character Set .......................... 72
14. Proposed Entities ..................................... 75
1. Introduction
The HyperText Markup Language (HTML) is a simple data format used to
create hypertext documents that are portable from one platform to
another. HTML documents are SGML documents with generic semantics
that are appropriate for representing information from a wide range
of domains.
As HTML is an application of SGML, this specification assumes a working knowledge of [SGML]. 1.1. Scope HTML has been in use by the World-Wide Web (WWW) global information initiative since 1990. Previously, informal documentation on HTML has been available from a number of sources on the Internet. This specification brings together, clarifies, and formalizes a set of features that roughly corresponds to the capabilities of HTML in common use prior to June 1994. A number of new features to HTML are being proposed and experimented in the Internet community. This document thus defines a HTML 2.0 (to distinguish it from the previous informal specifications). Future (generally upwardly compatible) versions of HTML with new features will be released with higher version numbers. HTML is an application of ISO Standard 8879:1986, "Information Processing Text and Office Systems; Standard Generalized Markup Language" (SGML). The HTML Document Type Definition (DTD) is a formal definition of the HTML syntax in terms of SGML. This specification also defines HTML as an Internet Media Type[IMEDIA] and MIME Content Type[MIME] called `text/html'. As such, it defines the semantics of the HTML syntax and how that syntax should be interpreted by user agents. 1.2. Conformance This specification governs the syntax of HTML documents and aspects of the behavior of HTML user agents. 1.2.1. Documents A document is a conforming HTML document if: * It is a conforming SGML document, and it conforms to the HTML DTD (see 9.1, "HTML DTD"). NOTE - There are a number of syntactic idioms that are not supported or are supported inconsistently in some historical user agent implementations. These idioms are identified in notes like this throughout this specification. * It conforms to the application conventions in this specification. For example, the value of the HREF attribute
of the <A> element must conform to the URI syntax.
* Its document character set includes [ISO-8859-1] and
agrees with [ISO-10646]; that is, each code position listed
in 13, "The HTML Coded Character Set" is included, and each
code position in the document character set is mapped to the
same character as [ISO-10646] designates for that code
position.
NOTE - The document character set is somewhat
independent of the character encoding scheme used to
represent a document. For example, the `ISO-2022-JP'
character encoding scheme can be used for HTML
documents, since its repertoire is a subset of the
[ISO-10646] repertoire. The critical distinction is
that numeric character references agree with
[ISO-10646] regardless of how the document is
encoded.
1.2.2. Feature Test Entities
The HTML DTD defines a standard HTML document type and several
variations, by way of feature test entities. Feature test entities
are declarations in the HTML DTD that control the inclusion or
exclusion of portions of the DTD.
HTML.Recommended
Certain features of the language are necessary for
compatibility with widespread usage, but they may
compromise the structural integrity of a document. This
feature test entity selects a more prescriptive document
type definition that eliminates those features. It is
set to `IGNORE' by default.
For example, in order to preserve the structure of a
document, an editing user agent may translate HTML
documents to the recommended subset, or it may require
that the documents be in the recommended subset for
import.
HTML.Deprecated
Certain features of the language are necessary for
compatibility with earlier versions of the
specification, but they tend to be used and implemented
inconsistently, and their use is deprecated. This
feature test entity enables a document type definition
that allows these features. It is set to `INCLUDE' by
default.
Documents generated by translation software or editing
software should not contain deprecated idioms.
1.2.3. User Agents
An HTML user agent conforms to this specification if:
* It parses the characters of an HTML document into data
characters and markup according to [SGML].
NOTE - In the interest of robustness and
extensibility, there are a number of widely deployed
conventions for handling non-conforming documents.
See 4.2.1, "Undeclared Markup Error Handling" for
details.
* It supports the `ISO-8859-1' character encoding scheme and
processes each character in the ISO Latin Alphabet No. 1 as
specified in 6.1, "The HTML Document Character Set".
NOTE - To support non-western writing systems, HTML
user agents are encouraged to support
`ISO-10646-UCS-2' or similar character encoding
schemes and as much of the character repertoire of
[ISO-10646] as is practical.
* It behaves identically for documents whose parsed token
sequences are identical.
For example, comments and the whitespace in tags disappear
during tokenization, and hence they do not influence the
behavior of conforming user agents.
* It allows the user to traverse (or at least attempt to
traverse, resources permitting) all hyperlinks from <A>
elements in an HTML document.
An HTML user agent is a level 2 user agent if, additionally:
* It allows the user to express all form field values
specified in an HTML document and to (attempt to) submit the
values as requests to information services.
2. Terms absolute URI a URI in absolute form; for example, as per [URL] anchor one of two ends of a hyperlink; typically, a phrase marked as an <A> element. base URI an absolute URI used in combination with a relative URI to determine another absolute URI. character An atom of information, for example a letter or a digit. Graphic characters have associated glyphs, whereas control characters have associated processing semantics. character encoding scheme A function whose domain is the set of sequences of octets, and whose range is the set of sequences of characters from a character repertoire; that is, a sequence of octets and a character encoding scheme determines a sequence of characters. character repertoire A finite set of characters; e.g. the range of a coded character set. code position An integer. A coded character set and a code position from its domain determine a character. coded character set A function whose domain is a subset of the integers and whose range is a character repertoire. That is, for some set of integers (usually of the form {0, 1, 2, ..., N} ), a coded character set and an integer in that set determine a character. Conversely, a character and a coded character set determine the character's code position (or, in rare cases, a few code positions). conforming HTML user agent A user agent that conforms to this specification in its processing of the Internet Media Type `text/html'.
data character
Characters other than markup, which make up the content
of elements.
document character set
a coded character set whose range includes all
characters used in a document. Every SGML document has
exactly one document character set. Numeric character
references are resolved via the document character set.
DTD
document type definition. Rules that apply SGML to the
markup of documents of a particular type, including a
set of element and entity declarations. [SGML]
element
A component of the hierarchical structure defined by a
document type definition; it is identified in a document
instance by descriptive markup, usually a start-tag and
end-tag. [SGML]
end-tag
Descriptive markup that identifies the end of an
element. [SGML]
entity
data with an associated notation or interpretation; for
example, a sequence of octets associated with an
Internet Media Type. [SGML]
fragment identifier
the portion of an HREF attribute value following the `#'
character which modifies the presentation of the
destination of a hyperlink.
form data set
a sequence of name/value pairs; the names are given by
an HTML document and the values are given by a user.
HTML document
An SGML document conforming to this document type
definition.
hyperlink
a relationship between two anchors, called the head and
the tail. The link goes from the tail to the head. The
head and tail are also known as destination and source,
respectively.
markup
Syntactically delimited characters added to the data of
a document to represent its structure. There are four
different kinds of markup: descriptive markup (tags),
references, markup declarations, and processing
instructions. [SGML]
may
A document or user interface is conforming whether this
statement applies or not.
media type
an Internet Media Type, as per [IMEDIA].
message entity
a head and body. The head is a collection of name/value
fields, and the body is a sequence of octets. The head
defines the content type and content transfer encoding
of the body. [MIME]
minimally conforming
HTML user agent
A user agent that conforms to this specification except
for form processing. It may only process level 1 HTML
documents.
must
Documents or user agents in conflict with this statement
are not conforming.
numeric character
reference
markup that refers to a character by its code position
in the document character set.
SGML document
A sequence of characters organized physically as a set
of entities and logically into a hierarchy of elements.
An SGML document consists of data characters and markup;
the markup describes the structure of the information
and an instance of that structure. [SGML]
shall
If a document or user agent conflicts with this
statement, it does not conform to this specification.
should
If a document or user agent conflicts with this
statement, undesirable results may occur in practice
even though it conforms to this specification.
start-tag
Descriptive markup that identifies the start of an
element and specifies its generic identifier and
attributes. [SGML]
syntax-reference
character set
A coded character set whose range includes all
characters used for markup; e.g. name characters and
delimiter characters.
tag
Markup that delimits an element. A tag includes a name
which refers to an element declaration in the DTD, and
may include attributes. [SGML]
text entity
A finite sequence of characters. A text entity typically
takes the form of a sequence of octets with some
associated character encoding scheme, transmitted over
the network or stored in a file. [SGML]
typical
Typical processing is described for many elements. This
is not a mandatory part of the specification but is
given as guidance for designers and to help explain the
uses for which the elements were intended.
URI
A Uniform Resource Identifier is a formatted string that
serves as an identifier for a resource, typically on the
Internet. URIs are used in HTML to identify the anchors
of hyperlinks. URIs in common practice include Uniform
Resource Locators (URLs)[URL] and Relative URLs
[RELURL].
user agent
A component of a distributed system that presents an
interface and processes requests on behalf of a user;
for example, a www browser or a mail user agent.
WWW
The World-Wide Web is a hypertext-based, distributed
information system created by researchers at CERN in
Switzerland. <URL:http://www.w3.org/>
3. HTML as an Application of SGML
HTML is an application of ISO 8879:1986 -- Standard Generalized
Markup Language (SGML). SGML is a system for defining structured
document types and markup languages to represent instances of those
document types[SGML]. The public text -- DTD and SGML declaration --
of the HTML document type definition are provided in 9, "HTML Public
Text".
The term "HTML" refers to both the document type defined here and the
markup language for representing instances of this document type.
3.1. SGML Documents
An HTML document is an SGML document; that is, a sequence of
characters organized physically into a set of entities, and logically
as a hierarchy of elements.
In the SGML specification, the first production of the SGML syntax
grammar separates an SGML document into three parts: an SGML
declaration, a prologue, and an instance. For the purposes of this
specification, the prologue is a DTD. This DTD describes another
grammar: the start symbol is given in the doctype declaration, the
terminals are data characters and tags, and the productions are
determined by the element declarations. The instance must conform to
the DTD, that is, it must be in the language defined by this grammar.
The SGML declaration determines the lexicon of the grammar. It
specifies the document character set, which determines a character
repertoire that contains all characters that occur in all text
entities in the document, and the code positions associated with
those characters.
The SGML declaration also specifies the syntax-reference character
set of the document, and a few other parameters that bind the
abstract syntax of SGML to a concrete syntax. This concrete syntax
determines how the sequence of characters of the document is mapped
to a sequence of terminals in the grammar of the prologue.
For example, consider the following document:
<!DOCTYPE html PUBLIC "-//IETF//DTD HTML 2.0//EN">
<title>Parsing Example</title>
<p>Some text. <em>*wow*</em></p>
An HTML user agent should use the SGML declaration that is given in
9.5, "SGML Declaration for HTML". According to its document character
set, `*' refers to an asterisk character, `*'.
The instance above is regarded as the following sequence of
terminals:
1. start-tag: TITLE
2. data characters: "Parsing Example"
3. end-tag: TITLE
4. start-tag: P
5. data characters "Some text."
6. start-tag: EM
7. data characters: "*wow*"
8. end-tag: EM
9. end-tag: P
The start symbol of the DTD grammar is HTML, and the productions are
given in the public text identified by `-//IETF//DTD HTML 2.0//EN'
(9.1, "HTML DTD"). The terminals above parse as:
HTML
|
\-HEAD
| |
| \-TITLE
| |
| \-<TITLE>
| |
| \-"Parsing Example"
| |
| \-</TITLE>
|
\-BODY
|
\-P
|
\-<P>
|
\-"Some text. "
|
\-EM
| |
| \-<EM>
| |
| \-"*wow*"
| |
| \-</EM>
|
\-</P>
Some of the elements are delimited explicitly by tags, while the
boundaries of others are inferred. The <HTML> element contains a
<HEAD> element and a <BODY> element. The <HEAD> contains <TITLE>,
which is explicitly delimited by start- and end-tags.
3.2. HTML Lexical Syntax
SGML specifies an abstract syntax and a reference concrete syntax.
Aside from certain quantities and capacities (e.g. the limit on the
length of a name), all HTML documents use the reference concrete
syntax. In particular, all markup characters are in the repertoire of
[ISO-646]. Data characters are drawn from the document character set
(see 6, "Characters, Words, and Paragraphs").
A complete discussion of SGML parsing, e.g. the mapping of a sequence of characters to a sequence of tags and data, is left to the SGML standard[SGML]. This section is only a summary. 3.2.1. Data Characters Any sequence of characters that do not constitute markup (see 9.6 "Delimiter Recognition" of [SGML]) are mapped directly to strings of data characters. Some markup also maps to data character strings. Numeric character references map to single-character strings, via the document character set. Each reference to one of the general entities defined in the HTML DTD maps to a single-character string. For example, abc<def => "abc","<","def" abc<def => "abc","<","def" The terminating semicolon on entity or numeric character references is only necessary when the character following the reference would otherwise be recognized as part of the name (see 9.4.5 "Reference End" in [SGML]). abc < def => "abc ","<"," def" abc < def => "abc ","<"," def" An ampersand is only recognized as markup when it is followed by a letter or a `#' and a digit: abc & lt def => "abc & lt def" abc &# 60 def => "abc &# 60 def" A useful technique for translating plain text to HTML is to replace each '<', '&', and '>' by an entity reference or numeric character reference as follows: ENTITY NUMERIC CHARACTER REFERENCE CHAR REF CHARACTER DESCRIPTION --------- ---------- ----------- --------------------- & & & Ampersand < < < Less than > > > Greater than NOTE - There are SGML mechanisms, CDATA and RCDATA declared content, that allow most `<', `>', and `&' characters to be entered without the use of entity references. Because these mechanisms tend to be used and implemented inconsistently, and because they conflict
with techniques for reducing HTML to 7 bit ASCII for
transport, they are deprecated in this version of HTML.
See 5.5.2.1, "Example and Listing: XMP, LISTING".
3.2.2. Tags
Tags delimit elements such as headings, paragraphs, lists, character
highlighting, and links. Most HTML elements are identified in a
document as a start-tag, which gives the element name and attributes,
followed by the content, followed by the end tag. Start-tags are
delimited by `<' and `>'; end tags are delimited by `</' and `>'. An
example is:
<H1>This is a Heading</H1>
Some elements only have a start-tag without an end-tag. For example,
to create a line break, use the `<BR>' tag. Additionally, the end
tags of some other elements, such as Paragraph (`</P>'), List Item
(`</LI>'), Definition Term (`</DT>'), and Definition Description
(`</DD>') elements, may be omitted.
The content of an element is a sequence of data character strings and
nested elements. Some elements, such as anchors, cannot be nested.
Anchors and character highlighting may be put inside other
constructs. See the HTML DTD, 9.1, "HTML DTD" for full details.
NOTE - The SGML declaration for HTML specifies SHORTTAG YES, which
means that there are other valid syntaxes for tags, such as NET
tags, `<EM/.../'; empty start tags, `<>'; and empty end-tags,
`</>'. Until support for these idioms is widely deployed, their
use is strongly discouraged.
3.2.3. Names
A name consists of a letter followed by letters, digits, periods, or
hyphens. The length of a name is limited to 72 characters by the
`NAMELEN' parameter in the SGML declaration for HTML, 9.5, "SGML
Declaration for HTML". Element and attribute names are not case
sensitive, but entity names are. For example, `<BLOCKQUOTE>',
`<BlockQuote>', and `<blockquote>' are equivalent, whereas `&' is
different from `&'.
In a start-tag, the element name must immediately follow the tag open
delimiter `<'.
3.2.4. Attributes In a start-tag, white space and attributes are allowed between the element name and the closing delimiter. An attribute specification typically consists of an attribute name, an equal sign, and a value, though some attribute specifications may be just a name token. White space is allowed around the equal sign. The value of the attribute may be either: * A string literal, delimited by single quotes or double quotes and not containing any occurrences of the delimiting character. NOTE - Some historical implementations consider any occurrence of the `>' character to signal the end of a tag. For compatibility with such implementations, when `>' appears in an attribute value, it should be represented with a numeric character reference. For example, `<IMG SRC="eq1.jpg" alt="a>b">' should be written `<IMG SRC="eq1.jpg" alt="a>b">' or `<IMG SRC="eq1.jpg" alt="a>b">'. * A name token (a sequence of letters, digits, periods, or hyphens). Name tokens are not case sensitive. NOTE - Some historical implementations allow any character except space or `>' in a name token. In this example, <img> is the element name, src is the attribute name, and `http://host/dir/file.gif' is the attribute value: <img src='http://host/dir/file.gif'> A useful technique for computing an attribute value literal for a given string is to replace each quote and white space character by an entity reference or numeric character reference as follows: ENTITY NUMERIC CHARACTER REFERENCE CHAR REF CHARACTER DESCRIPTION --------- ---------- ----------- --------------------- HT 	 Tab LF Line Feed CR Carriage Return SP   Space " " " Quotation mark & & & Ampersand
For example: <IMG SRC="image.jpg" alt="First "real" example"> The `NAMELEN' parameter in the SGML declaration (9.5, "SGML Declaration for HTML") limits the length of an attribute value to 1024 characters. Attributes such as ISMAP and COMPACT may be written using a minimized syntax (see 7.9.1.2 "Omitted Attribute Name" in [SGML]). The markup: <UL COMPACT="compact"> can be written using a minimized syntax: <UL COMPACT> NOTE - Some historical implementations only understand the minimized syntax. 3.2.5. Comments To include comments in an HTML document, use a comment declaration. A comment declaration consists of `<!' followed by zero or more comments followed by `>'. Each comment starts with `--' and includes all text up to and including the next occurrence of `--'. In a comment declaration, white space is allowed after each comment, but not before the first comment. The entire comment declaration is ignored. NOTE - Some historical HTML implementations incorrectly consider any `>' character to be the termination of a comment. For example: <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <HEAD> <TITLE>HTML Comment Example</TITLE> <!-- Id: html-sgml.sgm,v 1.5 1995/05/26 21:29:50 connolly Exp --> <!-- another -- -- comment --> <!> </HEAD> <BODY> <p> <!- not a comment, just regular old data characters ->
3.3. HTML Public Text Identifiers To identify information as an HTML document conforming to this specification, each document must start with one of the following document type declarations. <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> This document type declaration refers to the HTML DTD in 9.1, "HTML DTD". NOTE - If the body of a `text/html' message entity does not begin with a document type declaration, an HTML user agent should infer the above document type declaration. <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0 Level 2//EN"> This document type declaration also refers to the HTML DTD which appears in 9.1, "HTML DTD". <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0 Level 1//EN"> This document type declaration refers to the level 1 HTML DTD in 9.3, "Level 1 HTML DTD". Form elements must not occur in level 1 documents. <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0 Strict//EN"> <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0 Strict Level 1//EN"> These two document type declarations refer to the HTML DTD in 9.2, "Strict HTML DTD" and 9.4, "Strict Level 1 HTML DTD". They refer to the more structurally rigid definition of HTML. HTML user agents may support other document types. In particular, they may support other formal public identifiers, or other document types altogether. They may support an internal declaration subset with supplemental entity, element, and other markup declarations. 3.4. Example HTML Document <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <HTML> <!-- Here's a good place to put a comment. --> <HEAD> <TITLE>Structural Example</TITLE> </HEAD><BODY> <H1>First Header</H1> <P>This is a paragraph in the example HTML file. Keep in mind
that the title does not appear in the document text, but that
the header (defined by H1) does.</P>
<OL>
<LI>First item in an ordered list.
<LI>Second item in an ordered list.
<UL COMPACT>
<LI> Note that lists can be nested;
<LI> Whitespace may be used to assist in reading the
HTML source.
</UL>
<LI>Third item in an ordered list.
</OL>
<P>This is an additional paragraph. Technically, end tags are
not required for paragraphs, although they are allowed. You can
include character highlighting in a paragraph. <EM>This sentence
of the paragraph is emphasized.</EM> Note that the </P>
end tag has been omitted.
<P>
<IMG SRC ="triangle.xbm" alt="Warning: ">
Be sure to read these <b>bold instructions</b>.
</BODY></HTML>
4. HTML as an Internet Media Type
An HTML user agent allows users to interact with resources which have
HTML representations. At a minimum, it must allow users to examine
and navigate the content of HTML level 1 documents. HTML user agents
should be able to preserve all formatting distinctions represented in
an HTML document, and be able to simultaneously present resources
referred to by IMG elements (they may ignore some formatting
distinctions or IMG resources at the request of the user). Level 2
HTML user agents should support form entry and submission.
4.1. text/html media type
This specification defines the Internet Media Type [IMEDIA] (formerly
referred to as the Content Type [MIME]) called `text/html'. The
following is to be registered with [IANA].
Media Type name
text
Media subtype name
html
Required parameters
none
Optional parameters
level, charset
Encoding considerations
any encoding is allowed
Security considerations
see 10, "Security Considerations"
The optional parameters are defined as follows:
Level
The level parameter specifies the feature set used in
the document. The level is an integer number, implying
that any features of same or lower level may be present
in the document. Level 1 is all features defined in this
specification except those that require the <FORM>
element. Level 2 includes form processing. Level 2 is
the default.
Charset
The charset parameter (as defined in section 7.1.1 of
RFC 1521[MIME]) may be given to specify the character
encoding scheme used to represent the HTML document as a
sequence of octets. The default value is outside the
scope of this specification; but for example, the
default is `US-ASCII' in the context of MIME mail, and
`ISO-8859-1' in the context of HTTP [HTTP].
4.2. HTML Document Representation
A message entity with a content type of `text/html' represents an
HTML document, consisting of a single text entity. The `charset'
parameter (whether implicit or explicit) identifies a character
encoding scheme. The text entity consists of the characters
determined by this character encoding scheme and the octets of the
body of the message entity.
4.2.1. Undeclared Markup Error Handling
To facilitate experimentation and interoperability between
implementations of various versions of HTML, the installed base of
HTML user agents supports a superset of the HTML 2.0 language by
reducing it to HTML 2.0: markup in the form of a start-tag or end-
tag, whose generic identifier is not declared is mapped to nothing
during tokenization. Undeclared attributes are treated similarly. The
entire attribute specification of an unknown attribute (i.e., the
unknown attribute and its value, if any) should be ignored. On the
other hand, references to undeclared entities should be treated as
data characters.
For example:
<div class=chapter><h1>foo</h1><p>...</div>
=> <H1>,"foo",</H1>,<P>,"..."
xxx <P ID=z23> yyy
=> "xxx ",<P>," yyy
Let α & β be finite sets.
=> "Let α & β be finite sets."
Support for notifying the user of such errors is encouraged.
Information providers are warned that this convention is not binding:
unspecified behavior may result, as such markup does not conform to
this specification.
4.2.2. Conventional Representation of Newlines
SGML specifies that a text entity is a sequence of records, each
beginning with a record start character and ending with a record end
character (code positions 10 and 13 respectively) (section 7.6.1,
"Record Boundaries" in [SGML]).
[MIME] specifies that a body of type `text/*' is a sequence of lines,
each terminated by CRLF, that is, octets 13, 10.
In practice, HTML documents are frequently represented and
transmitted using an end of line convention that depends on the
conventions of the source of the document; frequently, that
representation consists of CR only, LF only, or a CR LF sequence.
Hence the decoding of the octets will often result in a text entity
with some missing record start and record end characters.
Since there is no ambiguity, HTML user agents are encouraged to infer
the missing record start and end characters.
An HTML user agent should treat end of line in any of its variations
as a word space in all contexts except preformatted text. Within
preformatted text, an HTML user agent should treat any of the three
common representations of end-of-line as starting a new line.
(page 20 continued on part 2)
