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element. The contains, 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").Berners-Lee & Connolly Standards Track [Page 12] RFC 1866 Hypertext Markup Language - 2.0 November 1995 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 conflictBerners-Lee & Connolly Standards Track [Page 13] RFC 1866 Hypertext Markup Language - 2.0 November 1995 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 `<'.Berners-Lee & Connolly Standards Track [Page 14] RFC 1866 Hypertext Markup Language - 2.0 November 19953.2.4. 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="/image.pl?url=https%3a%2f%2fwww.rfc-editor.org%2frfc%2f.%2f.%2f.%2feq1.jpg&f=jpg&w=240" alt="a>b">' should be written `<img src="/image.pl?url=https%3a%2f%2fwww.rfc-editor.org%2frfc%2f.%2f.%2f.%2feq1.jpg&f=jpg&w=240" alt="a>b">' or `<img src="/image.pl?url=https%3a%2f%2fwww.rfc-editor.org%2frfc%2f.%2f.%2f.%2feq1.jpg&f=jpg&w=240" 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 & & & AmpersandBerners-Lee & Connolly Standards Track [Page 15] RFC 1866 Hypertext Markup Language - 2.0 November 1995 For example:<img src="/image.pl?url=https%3a%2f%2fwww.rfc-editor.org%2frfc%2f.%2f.%2f.%2fimage.jpg&f=jpg&w=240" 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 ExampleBerners-Lee & Connolly Standards Track [Page 16] RFC 1866 Hypertext Markup Language - 2.0 November 19953.3. 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. 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. This document type declaration also refers to the HTML DTD which appears in 9.1, "HTML DTD". 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. 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
Structural ExampleFirst Header
This is a paragraph in the example HTML file. Keep in mindBerners-Lee & Connolly Standards Track [Page 17] RFC 1866 Hypertext Markup Language - 2.0 November 1995 that the title does not appear in the document text, but that the header (defined by H1) does.
- First item in an ordered list.
- Second item in an ordered list.
- Note that lists can be nested;
- Whitespace may be used to assist in reading the HTML source.
- Third item in an ordered list.
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.This sentence of the paragraph is emphasized. Note that the </P> end tag has been omitted.
Be sure to read thesebold instructions.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 noneBerners-Lee & Connolly Standards Track [Page 18] RFC 1866 Hypertext Markup Language - 2.0 November 1995 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