This section of the document introduces SGML and discusses its
relationship to HTML. A complete discussion of SGML is left to the standard
(see [ISO8879]).
SGML is a system for defining markup languages. Authors
mark
up their documents by representing structural, presentational,
and semantic information alongside content. HTML is one example of a markup
language. Here is an example of an HTML document:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<HTML>
<HEAD>
<TITLE>My first HTML document</TITLE>
</HEAD>
<BODY>
<P>Hello world!
</BODY>
</HTML>
An HTML document is divided into a head section (here, between <HEAD>
and </HEAD>) and a body (here, between <BODY> and </BODY>).
The title of the document appears in the head (along with other information
about the document), and the content of the document appears in the body. The
body in this example contains just one paragraph, marked up with <P>.
Each markup language defined in SGML is called an SGML
application. An SGML application is generally characterized
by:
A specification that describes the semantics to be ascribed to the markup.
This specification also imposes syntax restrictions that cannot be expressed
within the DTD.
Document instances containing data (content) and markup. Each instance
contains a reference to the DTD to be used to interpret it.
An SGML document type definition declares
element types that represent structures or desired behavior.
HTML includes element types that represent paragraphs, hypertext links, lists,
tables, images, etc.
Each element type
declaration generally describes three parts: a start tag,
content, and an end tag.
The element's name appears in the start
tag (written <element-name>) and the
end tag (written </element-name>);
note the slash before the element name in the end tag. For example, the start
and end tags of the UL element type delimit the items in a list:
Some HTML element types allow authors to
omit end tags (e.g., the P and LI
element types). A few element types also allow the
start tags to be omitted; for example,
HEAD and BODY. The HTML DTD indicates for each element
type whether the start tag and end tag are required.
Some HTML element types have no content. For example, the line break element
BR has no content; its only role is to terminate a line of text.
Such
empty elements never have end tags. The document type definition and the text of the
specification indicate whether an element type is empty (has no content) or, if
it can have content, what is considered legal content.
Please consult the SGML standard for information about rules governing
elements (e.g., they must be properly nested, an end tag closes, back to the
matching start tag, all unclosed intervening start tags with omitted end tags
(section 7.5.1), etc.).
For example, the following paragraph:
<P>This is the first paragraph.</P>
...a block element...
may be rewritten without its end tag:
<P>This is the first paragraph.
...a block element...
since the <P> start tag is closed by the following block element.
Similarly, if a paragraph is enclosed by a block element, as in:
<DIV>
<P>This is the paragraph.
</DIV>
the end tag of the enclosing block element (here, </DIV>) implies the
end tag of the open <P> start tag.
Elements are not tags. Some people refer to elements as
tags (e.g., "the P tag"). Remember that the element is one thing, and the tag
(be it start or end tag) is another. For instance, the HEAD element is always
present, even though both start and end HEAD tags may be missing in the
markup.
All the element types declared in this specification are listed in the element index.
Elements may have associated properties, called attributes,
which may have values (by default, or set by authors or scripts).
Attribute/value pairs appear before the final ">" of an element's start tag.
Any number of (legal) attribute value pairs, separated by spaces, may appear in
an element's start tag. They may appear in any order.
In this example, the id attribute is set for an
H1 element:
<H1 id="section1">
This is an identified heading thanks to the id attribute
</H1>
By default, SGML requires that all attribute values be delimited using either double quotation marks
(ASCII decimal 34) or single quotation marks (ASCII decimal 39). Single quote
marks can be included within the attribute value when the value is delimited by
double quote marks, and vice versa. Authors may also use numeric character references to represent double
quotes (") and single quotes ('). For double quotes authors can
also use the character entity reference
".
In certain cases, authors may specify the value of an attribute without any
quotation marks. The attribute value may only contain letters (a-z and A-Z),
digits (0-9), hyphens (ASCII decimal 45), periods (ASCII decimal 46),
underscores (ASCII decimal 95), and colons (ASCII decimal 58). We recommend
using quotation marks even when it is possible to eliminate them.
Attribute values are generally case-insensitive. The
definition of each attribute in the reference manual indicates whether its
value is case-insensitive.
All the attributes defined by this specification are listed in the attribute index.
Character references are
numeric or symbolic names for characters that may be included in an HTML
document. They are useful for referring to rarely used characters, or those
that authoring tools make it difficult or impossible to enter. You will see
character references throughout this document; they begin with a "&" sign
and end with a semi-colon (;). Some common examples include:
"<" represents the < sign.
">" represents the > sign.
""" represents the " mark.
"å" (in decimal) represents the letter "a" with a small circle
above it.
"И" (in decimal) represents the Cyrillic capital letter "I".
"水" (in hexadecimal) represents the Chinese character for
water.
<!-- this is a comment -->
<!-- and so is this one,
which occupies more than one line -->
White space is not permitted between the markup declaration open
delimiter("<!") and the comment open delimiter ("--"), but is permitted
between the comment close delimiter ("--") and the markup declaration close
delimiter (">"). A common error is to include a string of hyphens ("---")
within a comment. Authors should avoid putting two or more adjacent hyphens
inside comments.
Information that appears between comments has no special meaning (e.g., character references are not interpreted as
such).
Each element and attribute declaration in this specification is accompanied
by its document type definition fragment. We
have chosen to include the DTD fragments in the specification rather than seek
a more approachable, but longer and less precise means of describing an
element's properties. The following
tutorial should allow readers unfamiliar with SGML to read the DTD
and understand the technical details of the HTML specification.
The HTML DTD begins with a series of
parameter entity definitions. A parameter entity
definition defines a kind of macro that may be referenced and
expanded elsewhere in the DTD. These macros may not appear in HTML documents,
only in the DTD. Other types of macros, called
character references, may be used in the text of an HTML document or within
attribute values.
When the parameter entity is referred to by name in the DTD, it is expanded
into a string.
A parameter entity definition begins with the keyword <!ENTITY %
followed by the entity name, the quoted string the entity expands to, and
finally a closing >. Instances of parameter entities in a DTD begin
with "%", then the parameter entity name, and terminated by an optional
";".
The following example defines the string that the "%fontstyle;" entity will
expand to.
<!ENTITY % fontstyle "TT | I | B | BIG | SMALL">
The string the parameter entity expands to may contain other parameter
entity names. These names are expanded recursively. In the following example,
the "%inline;" parameter entity is defined to include the "%fontstyle;",
"%phrase;", "%special;" and "%formctrl;" parameter entities.
The bulk of the HTML DTD consists of the declarations of
element types and their attributes. The <!ELEMENT keyword
begins a declaration and the > character ends it. Between these are
specified:
The element's name.
Whether the element's tags are optional. Two
hyphens that appear after the element name mean that the start and end tags are
mandatory. One hyphen followed by the letter "O" indicates that the end tag can
be omitted. A pair of letter "O"s indicate that both the start and end tags can
be omitted.
The element's content, if any. The allowed content for an element is called
its content model. Element types
that are designed to have no content are called empty
elements. The content model for such element types is declared
using the keyword "EMPTY".
In this example:
<!ELEMENT UL - - (LI)+>
The element type being declared is UL.
The two hyphens indicate that both the start tag <UL> and the end tag
</UL> for this element type are required.
The content model for this element type is declared to be "at least one LI
element". Below, we explain how to specify content models.
This example illustrates the declaration of an empty element type:
<!ELEMENT IMG - O EMPTY>
The element type being declared is IMG.
The hyphen and the following "O" indicate that the end tag can be omitted,
but together with the content model "EMPTY", this is strengthened to the rule
that the end tag must be omitted.
The "EMPTY" keyword means that instances of this type must not have
content.
Content model definitions
The content model describes what may be contained by an instance of an
element type. Content model definitions may
include:
The names of allowed or forbidden element types (e.g., the UL
element contains instances of the LI element type, and the P
element type may not contain other P elements).
DTD entities (e.g., the LABEL element contains instances of the
"%inline;" parameter entity).
Document text (indicated by the SGML construct "#PCDATA"). Text may contain
character references. Recall that these
begin with & and end with a semicolon (e.g., "Hergé's adventures
of Tintin" contains the character entity reference for the "e acute"
character).
The content model of an element is specified with the following syntax.
Please note that the list below is a simplification of the full SGML syntax
rules and does not address, e.g., precedences.
(... )
Delimits a group.
A
A must occur, one time only.
A+
A must occur one or more times.
A?
A must occur zero or one time.
A*
A may occur zero or more times.
+(A)
A may occur.
-(A)
A must not occur.
A | B
Either A or B must occur, but not both.
A , B
Both A and B must occur, in that order.
A & B
Both A and B must occur, in any order.
Here are some examples from the HTML DTD:
<!ELEMENT UL - - (LI)+>
The UL element must contain one or more LI
elements.
<!ELEMENT DL - - (DT|DD)+>
The DL element must contain one or more DT
or
DD elements in any order.
<!ELEMENT OPTION - O (#PCDATA)>
The OPTION element may only contain text and entities, such as
& -- this is indicated by the SGML data type #PCDATA.
A few HTML element types use an additional SGML feature to exclude elements
from their content model. Excluded
elements are preceded by a hyphen. Explicit exclusions override
permitted elements.
In this example, the -(A) signifies that the element A
cannot appear in another A element (i.e., anchors may not be nested).
<!ELEMENT A - - (%inline;)* -(A)>
Note that the A element type is part of the DTD parameter entity
"%inline;", but is excluded explicitly because of -(A).
Similarly, the following element type declaration for
FORM prohibits nested forms:
The <!ATTLIST keyword begins the declaration of
attributes that an element may take. It is followed by the name of
the element in question, a list of attribute definitions, and a closing >.
Each attribute definition is a triplet that defines:
The name of an attribute.
The type of the attribute's value or an explicit set of possible values.
Values defined explicitly by the DTD are case-insensitive. Please consult the section on basic HTML data types for more information about
attribute value types.
Whether the default value of the attribute is implicit (keyword "#IMPLIED"), in
which case the default value must be supplied by the user agent (in some cases
via inheritance from parent elements); always required (keyword "#REQUIRED"); or
fixed to the given value
(keyword "#FIXED"). Some attribute definitions explicitly specify a default
value for the attribute.
In this example, the name attribute is defined for the
MAP element. The attribute is optional for this element.
<!ATTLIST MAP
name CDATA #IMPLIED
>
The type of values permitted for the attribute is given as CDATA, an SGML
data type.
CDATA is text that may contain
character references.
For more information about "CDATA", "NAME", "ID", and other data types,
please consult the section on HTML data types.
The following examples illustrate several attribute definitions:
rowspan NUMBER 1 -- number of rows spanned by cell --
http-equiv NAME #IMPLIED -- HTTP response header name --
id ID #IMPLIED -- document-wide unique id --
valign (top|middle|bottom|baseline) #IMPLIED
The rowspan attribute requires values of type NUMBER. The default
value is given explicitly as "1". The optional http-equiv attribute
requires values of type NAME. The optional id attribute requires
values of type ID. The optional valign attribute is constrained to
take values from the set {top, middle, bottom, baseline}.
DTD entities in attribute definitions
Attribute definitions may also contain parameter entity references.
In this example, we see that the attribute definition list for the
LINK element begins with the "%attrs;" parameter entity.
The "%coreattrs;" parameter entity in the "%attrs;" definition expands as
follows:
<!ENTITY % coreattrs
"idID #IMPLIED -- document-wide unique id --
classCDATA #IMPLIED -- space-separated list of classes --
style%StyleSheet; #IMPLIED -- associated style info --
title%Text; #IMPLIED -- advisory title --"
>
The "%attrs; parameter entity has been
defined for convenience since these attributes are defined for most HTML
element types.
Similarly, the DTD defines the "%URI;" parameter entity as expanding into
the string "CDATA".
<!ENTITY % URI "CDATA"
-- a Uniform Resource Identifier,
see [URI]
-->
As this example illustrates, the parameter entity "%URI;" provides readers
of the DTD with more information as to the type of data expected for an
attribute. Similar entities have been defined for "%Color;", "%Charset;",
"%Length;", "%Pixels;", etc.
Some attributes play the role of boolean variables (e.g., the selected attribute for the
OPTION element). Their appearance in the start tag of an element
implies that the value of the attribute is "true". Their absence implies a
value of "false".
Boolean attributes may legally take a single value: the name of the
attribute itself (e.g., selected="selected").
This example defines the selected attribute to be a
boolean attribute.
selected (selected) #IMPLIED -- option is pre-selected --
The attribute is set to "true" by appearing in the element's start tag:
In HTML, boolean attributes may appear in minimized form -- the
attribute's value appears alone in the element's start tag.
Thus, selected may be set by writing:
<OPTION selected>
instead of:
<OPTION selected="selected">
Authors should be aware that many user agents only
recognize the minimized form of boolean attributes and not the full form.
