Character Data
The keyword CDATA indicates that the content of an element is
character data. Character data is all the text up to the next end
tag open delimter-in-context. For example:
specifies that the following text is a legal XMP element:
<![CDATA[Here's an example. It looks like it has
<tags> and <!--comments-->
in it, but it does not. Even this
</ is data.]]>
The string There is no way to represent tags
in CDATA
Replaceable Character Data
Elements with RCDATA content behave much like thos with CDATA,
except for character references and entity references. Elements
declared like:
can have any sequence of characters in their content.
Character References
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To represent a character that would otherwise be recognized as
markup, use a character referece. The string &# signals a character
reference when it is followed by a letter or a digit. The delimiter
is followed by the decimal character number and a semicolon. For
example:
You can even represent </end> tags in RCDATA
Entity References
The HTML DTD declares entities for the less than, greater than, and
ampersand characters and each of the ISO Latin 1 characters so that
you can reference them by name rather than by number.
The string & signals an entity reference when it is followed by a
letter or a digit. The delimiter is followed by the entity name and
a semicolon. For example:
Kurt Gödel was a famous logician and mathemetician.
Note: To be sure that a string of characters has
no markup, HTML writers should represent all
occurences of <, >, and & by character or
entity references.
Element Content
Some elements have, in stead of a keyword that states the type of
content, a content model, which tells what patterns of data and
nested elements are allowed. If the content model of an element
does not include the symbol #PCDATA , the content is element
content.
Whitespace in element content is considered markup and ignored. Any
characters that are not markup, that is, data characters, are
illegal.
For example:
declares an element that may be used as follows:
Head Example
But the following are illegal:
no data allowed!
Two isindex tags
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Mixed Content
If the content model includes the symbol #PCDATA, the content of
the element is parsed as mixed content. For example:
This says that the PRE element contains one or more A, B, I, U, or
P elements or data characters. Here's an example of a PRE element:
NAME
cat -- concatenatefiles
EXAMPLE
cat
The content of the above PRE element is:
A B element
The string `` cat -- concatenate''
An A element
The string ``\n''
Another B element
The string ``\n cat . After the comment
delimiter, all text up to the next occurence of -- is ignored.
Hence comments cannot be nested. Whitespace is allowed between the
closing -- and >. (But not between the opening
HTML Guide: Recommended Usage
There are a few other SGML markup constructs that are deprecated or
illegal.
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Delimiter Signals...
.
.
LINE BREAKS
A line break character is considered markup (and ignored) if it is
the first or last piece of content in an element. This allows you
to write either
some example text
or
some example text
and these will be processed identically.
Also, a line that's not empty but contains no content will be
ignored altogether. For example, the element
first line
third line
fourth line
contains only the strings
first line
third line
fourth line.
SPACES AND TABS
Space characters must be rendered as horizontal white space. In
HTML, multiple spaces should be rendered as proportionally larger
spaces.
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The rendering of a horizontal tab (HT) character is not defined,
and HT should therefore not be used, except within a PRE (or
obsolete XMP, LISTING or PLAINTEXT) element.
Neither spaces nor tabs should be used to make SGML source layout
more attractive or easier to read.
SUMMARY OF MARKUP SIGNALS
The following delimiters may signal markup, depending on context.
Delimiter Signals
%ISOlat1;
-- Reference context for URLS --
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]>
LINK RELATIONSHIP VALUES
Status: This list is not part of the standard. It is intended to
illustrate the use of link relationships and to provide a framework
for further development.
Additions to this list will be controlled by the HTML registration
authority . Experimental values may be used on the condition that
they begin with "X-".
These values of the REL attribute of hypertext links have a
significance defined here, and may be treated in special ways by
HTML applications.
These relationships relate whole documents (objects), rather than
particular anchors within them. If the relationship value is used
with a link between anchors rather than whole documents, the
semantics are considered to apply to the documents.
In the explanations which follows, A is the source document of the
link and B is the destination document specified by the HREF
attribute.
A relationship marked "Acyclic" has the property that no sequence
of links with that relationship may be followed from any document
back to itself. These types of links may therefore be used to
define trees.
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Relationships between documents
These relationships are between the documents themselves rather
than the subjects of the documents.
USEINDEX
B is a related index for a search by a user reading this document
who asks for an index search function.
A document may have any number of index links, causing several
indexes top be searched in a client-defined manner.
B must support SEARCH operations under its access protocol.
USEGLOSSARY
B is an index which should be used to resolve glossary queries in
the document. (Typically, a double-click on a word which is not
within an anchor).
A document may have any number of glossary links.
ANNOTATION
The information in B is additional to and subsidiary to that in A.
Annotation is used by one person to write the equivalent of "margin
notes" or other criticism on another's document, for example.
Example: The relationship between a newsgroup and its articles.
Acyclic.
REPLY
Similar to Annotation, but there is no suggestion that B is
subsidiary to A: A and B are on equal footings.
Example: The relationship between a mail message and its reply, a
news article and its reply.
Acyclic.
EMBED
If this link is followed, the node at the end of it is embedded
into the display of the source document.
Acyclic.
PRECEDES
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In an ordered structure defined by the author, A precedes B, B is
followed by A.
Acyclic.
Any document may only have one link of this relationship, and/or
one link of the reverse relationship.
Note: May be used to control navigational aids, generate printed
material, etc. In conjunction with "subdocument", may be used to
define a tree such as a printed book made of hypertext document.
The document can only have one such tree.
SUBDOCUMENT
B is a lower part in the author's hierarchy to A. Acyclic. See
also Precedes.
PRESENT
Whenever A is presented, B must also be presented. This implies
that whenever A is retrieved, B must also be retrieved.
SEARCH
When the link is followed, the node B should be searched rather
than presented. That is, where the client software allows it, the
user should immediately be presented with a search panel and
prompted for text. The search is then performed without an
intermediate retrieval or presentation of the node B
SUPERSEDES
B is a previous version of A.
Acyclic.
HISTORY
B is a list of versions of A
A link reverse link must exist from B to A and to all other known
versions of A.
Relationships about subjects of documents
These relationships convey semantics about objects described by
documents, rather than the documents themselves.
INCLUDES
A includes B, B is part of A. For example, a person described by
document A is a part of the group described by document B.
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Acyclic.
MADE
Person (etc) described by node A is author of, or is responsible
for B
This information can be used for protection, and informing authors
of interest, for sending mail to authors, etc.
INTERESTED
Person (etc) described by A is interested in node B
This information can be used for informing readers of changes.
REGISTRATION AUTHORITY
The HTTP Registration Authority is responsible for maintaining
lists of:
Relationship names for link and anchor elements
It is proposed that the Internet Assigned Numbers Authority or
their successors take this role.
Unregistered values may be used for experimental purposes if they
are start with "X-".
REFERENCES
SGML ISO 8879:1986, Information ProcessingText
and Office SystemsStandard Generalized Markup
Language (SGML).
sgmls an SGML parser by James Clark
derived from the ARCSGML
parser materials which were written by
Charles F. Goldfarb. The source is available
on the ifi.uio.no FTP server in the directory
/pub/SGML/SGMLS .
WWW The World-Wide Web , a global information
initiative. For boostrap information, telnet
info.cern.ch or find documents by
ftp://info.cern.ch/pub/www/doc
URL Universal Resource Locators. RFCxxx.
Currently available by anonymous FTP from
info.cern.ch in /pub/ietf.
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AUTHOR'S ADDRESSES
This document was prepared with the help and advice of many people
across the net. Dan Connolly prepared the DTD and the section on
HTML and SGML whilst with Convex Computer Corporation of 3000
Waterview Parkway Richardson, TX 75083. He is now with Atrium
Technology Inc., and is not a current editor of the document.
Tim Berners-Lee
Address CERN
1211 Geneva 23
Switzerland
Telephone: +41(22)767 3755
Fax: +41(22)767 7155
email: timbl@info.cern.ch
Daniel Connolly
Address: Atrium Technologies, Inc.
5000 Plaza on the Lake, Suite 275
Austin, TX 78746
USA
email: connolly@atrium.com
Berners-Lee and Connolly 40