An HTML 4.0 document generally consists of three parts: a line containing version information, a descriptive header section, and a body, which contains the document's actual content.
The SGML DOCTYPE construct declares which version of HTML was used in composing the document (see [GOLD90]).
Authors should include a declaration resembling the following as the first line of each document:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Draft//EN">
The last two letters of the declaration indicate the language of the HTML DTD, in this case English ("EN"). User agents may ignore this information.
Authors may employ a different document type description depending on the version of HTML their document relies on. Recommended document types for HTML 4.0 are:
For documents following the draft specification for HTML 4.0
For documents following the final specification for HTML 4.0
For documents following the strict specification for HTML 4.0. This is appropriate when you wish to validate documents that don't use the HTML presentation elements and attributes, such as the FONT element and the align attribute.
For validatation against the DTD on the W3C Web site
For validation against the strict DTD on the W3C Web site
The binding between public identifiers and files can be specified using a catalog file following the format recommended by the SGML Open Consortium. A sample catalog file for HTML 4.0 is included at the beginning of the section on SGML reference information for HTML.
<!ENTITY % version "version CDATA #FIXED '%HTML.Version;'"> <!ENTITY % html.content "HEAD, (FRAMESET|BODY)"> <!ELEMENT HTML O O (%html.content)> <!ATTLIST HTML %version; %i18n; -- lang, dir -- >
Start tag: optional, End tag: optional
Attributes defined elsewhere
After version information, the remainder of an HTML document should be enclosed by the HTML element. Thus, a typical HTML document has this structure:
<!DOCTYPE HTML PUBLIC -//W3C//DTD HTML 4.0 Draft//EN> <HTML> ...The head, body, etc. goes here... </HTML>
<!-- %head.misc defined earlier on as "SCRIPT | STYLE | META | LINK" --> <!ENTITY % head.content "TITLE & ISINDEX? & BASE?"> <!ELEMENT HEAD O O (%head.content) +(%head.misc)> <!ATTLIST HEAD %i18n; -- lang, dir -- profile %URL #IMPLIED -- named dictionary of meta info -- >
Start tag: optional, End tag: optional
Attribute definitions
Attributes defined elsewhere
The HEAD element contains information about the current document, such as its title, keywords that may be useful to search engines, and other data that is not considered document content. Elements within the HEAD declaration must not be rendered by conforming user agents unless otherwise specified.
<!ELEMENT TITLE - - (#PCDATA)* -(%head.misc) -- The TITLE element is not considered part of the flow of text. It should be displayed, for example as the page header or window title. Exactly one title is required per document. --> <!ATTLIST TITLE %i18n>
Start tag: required, End tag: required
Attributes defined elsewhere
Every HTML document must have exactly one TITLE element in the HEAD section. User agents generally use the title to give people some idea about the document's contents, for example, by displaying the title as a caption, or speaking it.
Titles may contain character entities (for accented characters, special characters, etc.), but may not contain other markup. Here is a sample document title:
<HTML> <HEAD> <TITLE>A study of population dynamics</TITLE> ... other head elements... </HEAD> <BODY> ... document body... </BODY> </HTML>
Related to the TITLE element is the title attribute.
...some text... Here's a photo of <A href="http://someplace.com/neatstuff.gif" title="Me scuba diving"> me scuba diving last summer </A> ...some more text...
The title attribute has an additional role when used with the LINK element to designate an external style sheet. Please consult this section for details.
Note: To improve the quality of speech synthesis for cases handled poorly by standard techniques, future versions of HTML may include an attribute for encoding phonemic and prosodic information.
As this specification is being written, a number of approaches are being proposed for allowing authors to assign richer machine-readable information about documents and other network-accessible resources to an HTML document.
The current HTML specification allows authors to assign meta data to their documents as follows:
Note that since a profile is defined for the HEAD element, the same profile applies to all META and LINK elements in the document head.
<!ELEMENT META - O EMPTY -- Generic Metainformation --> <!ATTLIST META %i18n; -- lang, dir, for use with content string -- http-equiv NAME #IMPLIED -- HTTP response header name -- name NAME #IMPLIED -- metainformation name -- content CDATA #REQUIRED -- associated information -- scheme CDATA #IMPLIED -- select form of content -- >
Start tag: required, End tag: forbidden
Attribute definitions
For the following attributes, the permitted values and their interpretation are profile dependent:
Attributes defined elsewhere
The META element can be used to describe properties of a document (e.g., author, expiration date, a list of key words, etc.) and assign values to those properties. This specification does not define a normative set of properties.
The name attribute specifies a property and the content attribute specifies the property's value. For example,
<META name="Author" content="Dave Raggett">
The lang attribute can be used with META to specify the language for the value of the content attribute. This enables speech synthesisers to apply language dependent pronunciation rules.
In this example, the author's name is declared to be French.
<META name="Author" lang="fr" content="Arnaud Le Hors">
Here's another example: some user agents support the use of META to refresh the current page after a few seconds, perhaps replacing it with another page.
<META name="refresh" content="3,http://www.acme.com/intro.html">
The content is a number specifying the delay in seconds, followed by the URL to load when the time is up. This mechanism is generally used to show users a fleeting introductory page. However, since some user agents do not support this mechanism, authors should include content on the introductory page to allow users to navigate away from it (so they don't remain "stranded" on the introductory page).
The http-equiv attribute can be used in place of the name attribute and has a special significance when documents are retrieved via the Hypertext Transfer Protocol (HTTP). HTTP servers may use the property name specified by the http-equiv attribute to create an [RFC822]-style header in the HTTP response. Please see the HTTP specification ([RFC2068]) for details on valid HTTP headers.
The following sample META declaration:
<META http-equiv="Expires" content="Tue, 20 Aug 1996 14:25:27 GMT">
will result in the HTTP header:
Expires: Tue, 20 Aug 1996 14:25:27 GMT
This can be used by caches to determine when to fetch a fresh copy of the associated document.
A common use for META is to specify keywords that a search engine may use to improve the quality of search results. When several META elements provide language-dependent information about a document, search engines may filter on the lang attribute to display search results using the language preferences of the user. For example,
<META name="keywords" lang="en"
content="vacation,Greece,sunshine">
<META name="keywords" lang="fr"
content="vacances,Grèce,soleil">
The effectiveness of search engines can also be increased by using the LINK element to specify links to translations of the document in other languages, links to versions of the document in other media (e.g., PDF), and, when the document is part of a collection, links to an appropriate starting point for browsing the collection.
This example illustrates how one can use a META declaration to include a PICS 1.1 label:
<HEAD>
<META http-equiv="PICS-Label" content='
(PICS-1.1 "http://www.gcf.org/v2.5"
labels on "1994.11.05T08:15-0500"
until "1995.12.31T23:59-0000"
for "http://w3.org/PICS/Overview.html"
ratings (suds 0.5 density 0 color/hue 1))
'>
<TITLE>..title goes here..</TITLE>
</HEAD>
<BODY>
...the body...
</BODY>
The META element may be used to specify the default information for a document in the following instances:
The following example specifies the character encoding for a document as being ISO-8859-5
<META http-equiv="Content-Type" content="text/html; charset=ISO-8859-5">
This example refers to a hypothetical profile that defines useful properties for document indexing. The properties defined by this profile --- including "author", "copyright", "keywords", and "date" --- have their values set by subsequent META declarations.
<HEAD profile="http://www.acme.com/profiles/core"> <TITLE>How to complete Memorandum cover sheets</TITLE> <META name="author" content="John Doe"> <META name="copyright" content="© 1997 Acme Corp."> <META name="keywords" content="corporate,guidelines,cataloging"> <META name="date" content="23 Jan 1997 16:05:31 GMT"> </HEAD>
As this specification is being written, it is common practice to use the date formats described in [RFC2068]. HTTP applications have historically allowed three different formats for the representation of date/time stamps:
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123 Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036 Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
According to [RFC2068], the first format is preferred. It represents a fixed-length subset of that defined by [RFC1123] (an update to [RFC822]). The second format is in common use, but is based on the obsolete [RFC850]date format and lacks a four-digit year. HTTP 1.1 clients must accept all of these forms, but only generate [RFC1123] format for use in HTTP headers. HTML user agents are expected to follow HTTP 1.1 in this regards, and in addition to provide support for the [ISO8601] date format, e.g. "1997-01-23T16:05:31+00:00". For more information, see the sections on the INS and DEL elements.
The scheme attribute is used to identify the expected format of the value of the content attribute, for cases when a property supports multiple formats. The values permitted for the scheme attribute depend on the property name and the profile.
The first META declaration in the following example refers to the Dewey Decimal System (dds) scheme. The second refers to the ISBN scheme.
<META scheme="dds" name="description"
content="04.251 Supercomputers systems design">
<META scheme="ISBN" name="identifier" content="0-8230-2355-9">
Note: One sample profile is the Dublin Core[DCORE]. This profile defines a set of recommended properties for electronic bibliographic descriptions, and is intended to promote interoperability among disparate description models.
<!ENTITY % block "(%blocklevel | %inline)*">
<!ENTITY % Color "CDATA" -- a color using sRGB: #RRGGBB as Hex values -->
<!-- There are also 16 widely known color names with their sRGB values:
Black = #000000 Green = #008000
Silver = #C0C0C0 Lime = #00FF00
Gray = #808080 Olive = #808000
White = #FFFFFF Yellow = #FFFF00
Maroon = #800000 Navy = #000080
Red = #FF0000 Blue = #0000FF
Purple = #800080 Teal = #008080
Fuchsia= #FF00FF Aqua = #00FFFF
-->
<!ENTITY % bodycolors "
bgcolor %Color #IMPLIED
text %Color #IMPLIED
link %Color #IMPLIED
vlink %Color #IMPLIED
alink %Color #IMPLIED
">
<!ELEMENT BODY O O (%block) -(BODY) +(INS|DEL)>
<!ATTLIST BODY
%attrs; -- %coreattrs, %i18n, %events --
background %URL #IMPLIED -- texture tile for document background --
%bodycolors; -- bgcolor, text, link, vlink, alink --
onload %Script #IMPLIED -- the document has been loaded --
onunload %Script #IMPLIED -- the document has been removed --
>
Start tag: optional, End tag: optional
Attribute definitions
Attributes defined elsewhere
The body of a document contains the document's content. The content may be presented by a user agent in a variety of ways. For example, for visual browsers, you can think of the body as a canvas where the content appears: text, images, colors, graphics, etc. For audio user agents, the same content may be spoken. Since style sheets are now the preferred way to specify a document's presentation, the presentational attributes of BODY have been deprecated. These attributes should only be used when you need to control the presentation for user agents that don't support style sheets.
The following line of HTML illustrates the use of the deprecated attributes. It sets the background color of the canvas to white, the text foreground color to black, and the color of hyperlinks to red initially, fuschia when activated, and maroon once visited.
DEPRECATED EXAMPLE:
<HTML> <HEAD> <TITLE>A study of population dynamics</TITLE> </HEAD> <BODY bgcolor="white" text="black" link="red" alink="fuschia" vlink="maroon"> ... document body... </BODY> </HTML>
Using style sheets, the same effect could be accomplished as follows:
<HTML>
<HEAD>
<TITLE>A study of population dynamics</TITLE>
<STYLE type="text/css">
BODY { background: white; color: black}
A:link { color: red }
A:visited { color: maroon }
A:active { color: fuschia }
</STYLE>
</HEAD>
<BODY>
... document body...
</BODY>
</HTML>
Using linked style sheets gives you the flexibility to change the presentation without revising the document:
<HTML> <HEAD> <TITLE>A study of population dynamics</TITLE> <LINK rel="stylesheet" type="text/css" href="smartstyle.css"> </HEAD> <BODY> ... document body... </BODY> </HTML>
Framesets and HTML bodies. Documents that contain framesets replace the BODY element by the FRAMESET element. Please consult the section on frames for more information.
Attribute definitions
The id and class attributes assign identifiers to an element instance.
An identifier specified by id must be unique within a document. A class name specified by class may be shared by several element instances. Class values should be chosen to distinguish the role of the element the class is associated with, e.g. note, example, warning.
These attributes can be used in the following ways:
Almost every HTML element may be assigned identifier and class information.
Suppose, for example, that we are writing a document about a programming language. The document is to include a number of preformatted examples. We use the PRE element to format the examples. We also assign a background color (green) to all instances of the PRE element belonging to the class "example".
<HEAD>
<STYLE
PRE.example { background : green }
</STYLE
</HEAD>
<BODY>
<PRE class="example" id="example-1">
...example code here...
</PRE>
</BODY>
By setting the id attribute for this example, we can (1) create a hyperlink to it and (2) override class style information with instance style information.
<!ELEMENT DIV - - %block> <!ATTLIST DIV %attrs; -- %coreattrs, %i18n, %events -- %align; -- align, text alignment -- > <!ELEMENT SPAN - - (%inline)* -- generic language/style container --> <!ATTLIST SPAN %attrs; -- %coreattrs, %i18n, %events -- >
Start tag: required, End tag: required
Attributes defined elsewhere
The DIV and SPAN elements, in conjunction with the id and class attributes, offer a generic mechanism for adding structure to documents. These are the only two HTML elements that do not add presentation to their enclosed content. Thus, by creating instances and classes of elements and applying style sheets to them, authors may specialize HTML according to their needs and tastes.
Suppose we wanted to generate a document from a database of client information. Since HTML does not include elements that identify objects such as "client", "telephone number", "email address", etc., we use DIV and SPAN to tailor HTML to our own needs.
In this example, every client's last name belongs to the class "client-last-name", etc. We also assign a unique identifier to each client ("client-boyera", "client-lafon", etc.).
<DIV id="client-boyera" class="client"> <SPAN class="client-last-name">Last name:</SPAN> Boyera, <SPAN class="client-first-name">First name:</SPAN> Stephane <SPAN class="client-tel">Telephone:</SPAN> (212) 555-1212 <SPAN class="client-email">Email:</SPAN> sb@foo.org </DIV> <DIV id="client-lafon" class="client"> <SPAN class="client-last-name">Last name:</SPAN> Lafon, <SPAN class="client-first-name">First name:</SPAN> Yves <SPAN class="client-tel">Telephone:</SPAN> (617) 555-1212 <SPAN class="client-email">Email:</SPAN> yves@bar.com </DIV>
Later, we may easily add style information to our document to fine tune the presentation of these database entries.
SPAN is an inline element and can be used within paragraphs, list items, etc. when you want assign class or language information to a group of words. SPAN cannot be used to group block-level elements. SPAN has no inherent effect on rendering until you apply a style, e.g., via a style attribute, or a linked style sheet.
DIV by contrast, is a block-level element. It can be used to group other block-level elements, but can't be used within paragraph elements. A DIV element following an unclosed P element will terminate that paragraph.
User agents generally place a line break before and after DIV elements, for instance:
<P>aaaaaaaaa<DIV>bbbbbbbbb</DIV><DIV>ccccc<P>ccccc</DIV>
This is typically rendered as:
aaaaaaaaa bbbbbbbbb ccccc ccccc
Your user agent renders this as follows:
aaaaaaaaa
ccccc
<!ENTITY % heading "H1|H2|H3|H4|H5|H6"> <!-- There are six levels of headings from H1 (the most important) to H6 (the least important). --> <!ELEMENT (%heading) - - (%inline;)*> <!ATTLIST (%heading) %attrs; -- %coreattrs, %i18n, %events -- %align; -- align, text alignment -- >
Start tag: required, End tag: required
Attributes defined elsewhere
A heading element briefly describes the topic of the section it introduces. Heading information may be used by user agents, for example, to construct a table of contents for a document automatically.
There are six levels of headings in HTML with H1 as the most important and H6 as the least. Visual browsers usually render more important headings in larger fonts than less important ones.
The following example shows how to use the DIV element to associate a heading with the document section that follows it. Doing so allows you to define a style for the section (color the background, set the font, etc.) with style sheets.
<DIV class="section" id="forest-elephants" > <H1>Forest elephants</H1> In this section, we discuss the lesser known forest elephants. ...this section continues... <DIV class="subsection" id="forest-habitat" > <H2>Habitat</H2> Forest elephants do not live in trees but among them. ...this subsection continues... </DIV> </DIV>
This structure may be decorated with style information such as:
<HEAD>
<STYLE>
DIV.section { text-align: justify; font-size: 12pt}
DIV.subsection { text-indent: 2em }
H1 { font-style: italic; color: green }
H2 { color: green }
</STYLE>
</HEAD>
Numbered sections and references
HTML does
not itself cause section numbers to be generated from headings. This
facility may be offered by user agents, however. Soon, style sheet
languages such as CSS will allow authors to control the generation of
section numbers (handy for forward references in printed documents, as
in "See section 7.2").
Some people consider skipping heading levels to be bad practice. They accept H1 H2 H1 while they do not accept H1 H3 H1 since the heading level H2 is skipped.
<!ELEMENT ADDRESS - - ((%inline;) | P)*> <!ATTLIST ADDRESS %attrs; -- %coreattrs, %i18n, %events -- >
Start tag: required, End tag: required
Attributes defined elsewhere
For lack of a better place, we include the definition of the ADDRESS here. This element adds author and contact information to a document, e.g.,
<ADDRESS> Newsletter editor<BR> J. R. Brown<BR> 8723 Buena Vista, Smallville, CT 01234<BR> Tel: +1 (123) 456 7890 </ADDRESS>