Re: Is there a paper which describes the www protocol?
Tim Berners-Lee (timbl)
Thu, 9 Jan 92 12:34:24 GMT+0100
Date: Thu, 9 Jan 92 12:34:24 GMT+0100
From: timbl (Tim Berners-Lee)
Message-Id: <9201091134.AA08666@ nxoc01.cern.ch >
To: Mark Alexander Davis-Craig <mad@merit.edu>
Subject: Re: Is there a paper which describes the www protocol?
Cc: www-talk
> From: Mark Alexander Davis-Craig <mad@merit.edu>
>
> I was looking through the web and found information on servers and
> clients. I saw mention in the "History" section about wanting to
> develop a good protocol for information exchange, but haven't seen a
> paper specifically about the www protocol. Is there one? If not,
> could you describe it in some detail?
You are right that the protocol documentation was not as good as it could
have been. I have improved it. To save you browing through the web for it,
I append to this message the information as plain text.
> I ask because we at the University of Michigan are evaluating www,
> wais, and gopher for campus-wide information delivery.
>
I have no need to tell you what our suggestion would be! The W3 architecure
will give you (almost) everything you can get from WAIS and Gopher rolled into one.
The trick is that almost anything is representable by hypertext links and index searches. The
Gopher menus and plain text, for example, are both special cases of hypertext. As it is more
work to do the job for hypertext in general, we do not yet have software to cover as many
platforms as Gopher, for example. However, when we do, the W3 system will be more flexible.
Running a W3 server on top of a WAIS or Gopher world in fact makes these worlds subsets of the W3
web. The reverse is not possible because the WAIS and Gopher information models are not flexible
enough
to encompass the W3 model.
That said, if you want an indexer we can only recommend the wais code (or NeXT code) and we do
not yet supply (as Gopher does) an off-the shelf index server for either of those indexes yet. It
is easy to do, however, with our generic server code.
Please keep me informed of your thinking, whether you plan to go W3 or Gopher. If we can help
you set up a demonstration system, then mail me.
>Thanks in advance.
> -----------------------------------------------------------------
> Mark Davis-Craig, Merit/MichNet Technical Support Consultant
> mad@merit.edu mad@merit.bitnet (313)-936-2110
Tim Berners-Lee timbl@info.cern.ch
World Wide Web project (NeXTMail is ok)
CERN Tel: +41(22)767 3755
1211 Geneva 23, Switzerland Fax: +41(22)767 7155
_________________________________ protocol notes follow ___________
The HTTP Protocol As Implemented In W3
HTTP AS IMPLEMENTED IN WWW
This document defines the Hypertext Transfer protocol (HTTP) as currently
implemented by the WorldWideWeb initaitive software. This is a subset of the
proposed full HTTP protocol. No client profile information is transferred
with the query. Future HTTP protocols will be back-compatible with this
protocol.
The protocol uses the normal internet-style telnet protocol style on a
TCP-IP link. The following describes how a client acquires a (hypertext)
document from an HTTP server, given an HTTP document address .
Connection
The client makes a TCP-IP connection to the host using the domain name or IP
number , and the port number given in the address.
During development, the default HTTP TCP port number is 2784 -- this will
change when an official port number is allocated.
The server accepts the connection.
Note: HTTP currently runs over TCP, but could run over any
connection-oriented service. The interpretation of the protocol below in
the case of a sequenced packet service (such as DECnet(TM) or ISO TP4) is
that that the request should be one TPDU, but the repose may be many.
Request
The client sends a document request consisting of a line of ASCII characters
terminated by a CR LF (carriage return, line feed) pair. A well-behaved
server will not require the carriage return character.
This request consists of the word "GET", a space, the document address ,
omitting the "http:, host and port parts when they are the coordinates just
used to make the connection. (If a gateway is being used, then a full
document address may be given specifying a different naming scheme).
The search functionality of the protocol lies in the ability of the
addressing syntax to describe a search on a named index .
A search should only be requested by a client when the index document
itself has been descibed as an index using the ISINDEX tag .
Response
The response to a simple GET request is a message in hypertext mark-up
language ( HTML ). This is a byte stream of ASCII characters.
Lines shall be delimited by an optional carriage return followed by a
mandatory line feed chararcter. The client should not assume that the
carriage return will be present. Lines may be of any length. Well-behaved
servers should retrict line length to 80 characters excluding the CR LF
pair.
The format of the message is HTML - that is, a trimmed SGML document. Note
that this format allows for menus and hit lists to be returned as hypertext.
It also allows for plain ASCII text to be returned following the PLAINTEXT
tag .
The message is terminated by the closing of the connection by the server.
Well-behaved clients will read the entire document as fast as possible. The
client shall not wait for user action (output paging for example) before
reading the whole of the document. The server may impose a timeout of the
order of 15 seconds on inactivity.
Error responses are supplied in human readable text in HTML syntax. There
is no way to distinguish an error response from a satisfactory response
except for the content of the text.
Disconnection
The TCP-IP connection is broken by the server when the whole document has
been transferred.
The client may abort the transfer by breaking the connection before this,
in which case the server will not record any error condidtion.
Requests are idempotent . The server need not store any information about
the request after disconnection.
_________________________________________________________________
Tim BL
W3 NAMING SCHEMES
(See also: a discussion of design issues involved , BNF syntax , W3
background)
The format of a hypertext name consists of the name of the naming
sub-scheme to be used, then a name in a format particular to that subscheme,
then an optional anchor identifier within the document. For example, the
format is for all internet-based access methods:
scheme : // host.domain:port / path / path # anchor
A suffix # anchor id allows one to refer to a particular anchor within a
document.
A suffix ? followed by words separated by + signs allows one to seach an
index (see details ).
References from one document to another with a similar name may be
abbreviated to a relative name . This imposes certain restrictions on the
way that the "path" is represented.
A special format is used to represent a search on an index . See also: the
full BNF description , about escaping illegal characters .
Examples
file://cernvax.cern.ch/usr/lib/WWW/defaut.html#123
This is a fully qualified file name, referring to a document in the file
name space of the given internet node, and an imaginary anchor 123 within
it.
#greg
This refers to anchor "greg" in the same document as that in which the name
appears.
Naming sub-schemes
Different schemes usually use different protocols on the network. The format
of the address after the scheme name is a function of the particular scheme.
In practice, all internet-based schemes have a common format for the node
name and port. Schemes currently defined are as follows, with links to
more details.
file Access is provided to files, using whatever means the
browser and/or gateways have to reach files on obscure
machines.
news Access is provided to news articles, and newsgroups,
normally using the NNTP protocol.
http Access is provided to any other information using the
HTTP search and retrieve protocol . The internal
addressing of the information system is mapped onto a
W3 path.
telnet Access is provided by an interactive telnet session.
This is provided ONLY as an interface to other
existing online systems which cannot or have not been
mapped onto the W3 space.
gopher Access is provided using the "gopher" protocol. The
gopher protocol is similar to HTTP but uses separate
concepts of menus and text files rather than
hypertext.
Other schemes we foresee are wais and x500. Systems (such as WAIS) which
are not currently accessed directly be W3 servers may be accessed though
gateways, in which case the document address is encoded within the http
address of the document in the gateway. Browsers which do not have the
ability to use certain protocols may (in principle) be configured to
automaticaly use certain gateways for certain addressing schemes.
This will allow, for example, simple PC-based clients to follow links
through X500 name servers.
RELATIVE NAMING
The address of a hypertext document is normally given within the context of
another hypertext document. Where the addresses of the two documents are the
similar, this allows only the difference between the two names to be given,
saving space. An example is the address of the destination of a hypertext
link , which is specified relative to the source document address.
(A futher practical advantage is that a group of documents may be
transmitted without internal changes, or accessed using more than one
address.)
In the WWW address format , the rules for relative naming are:
If the "scheme" parts are different, the whole absolute address must be
given. Other wise, the scheme is omitted, and:
If the "host" and/or "port" parts are the different, the host name and
all the rest of the address must be given. The host name may be given
using internet hostname conventions, ie domains may be omitted where
different. This is not very well defined: one tends to assume that
if any dot is present, then the full domain name is being given, up
to the root (.) domain, while if there are no dots, the domain is the
same as that of the hostname part of the the base address.
If the access and host parts are the same, then the path may be given
with the unix convention, including the use of ".." to mean indicate
deletion of a path element. Within the path:
If a leading slash is present, the path is absolute. Otherwise:
The last part of the path of the base address (e.g. the filename of the
current document) is removed, and the given relative address appended
in its place.
Within the result, all occurences "xxx/.." are recursively removed,
where xxx is one path element (directory).
The use of the slash "/" and double dot ".." in this case must be respected
by all servers. If necessary, this may mean converting their local
representations in order that these characters should not appear within path
elements (see "escaping").
ADDRESS FOR AN INDEX SEARCH
If a given hypertext node is an index, or the server has an index associated
with it, then a search may be done on that index by suffixing the name of
the index with a list of keywords, after a question mark:
address_of_index ? keywordlist
The address of the index is a normal hypertext address. In the keuwordlist,
multiple keywords are separated by plus signs (+) . (See BNF syntax
description .) The resulting string still does not contain any spaces. It
may be considered to be the hypertext address of a document which is the
result of making the keyword search on the index. Normally, if the search
was successful, the document returned will contain anchors leading to other
documents which match the selection criteria.
The search method, and the logical and lexical functions, weights, etc
applied to the keywords will depend on the index address. One actual index
may have several hypertext addresses, which when searched on will behave in
different ways. For example, one may allow a search on author-given keywords
only, while another may be a full text search. These things particular to
an index should be descibed in the hypertext page for the index node itself
(or in linked documents). For example, a server may allow specific boolean
search combinations may be represented by the words "and", "or" and "not".
Example:
http://cernvm/FIND/?sgml+cms
indicates the result of perfoming a search for keywords "sgml" and "cms" on
the index http://cernvm/FIND/.
HTTP ADDRESSING
With an access code of http:, a protocol introduced for the WWW initiative
is used to acquire data from a server. This is the "Hypertext Transfer
protocol", HTTP , a simple search and retrieve (S and R) protocol.
The syntax of an http address is, with [] indicating optional parts (see
BNF description ),
http : // hostname [ : port ] / path [ ? searchwords ]
for example, the following are valid addresses:
http://info.cern.ch/hypertext/WWW/TheProject.html
http://crnvmc.cern.ch/FIND?sgml+examples
HTTP addresses conform to the WWW conventions, including the possibility of
using the search format . The significance of the items in the path part of
the document name is completely up to the server. Different paths may be
used to select different databases, different views of the same database,
etc.
hostname This is the name of the server in internet form. A
numeric form (e.g. 128.141.201.74) may be used, by the
domain name form (e.g. info.cern.ch) is preferred. The
hostname is mandatory.
port This is a numeric port number. If a non-numeric
string is used, it must be a defined service name.
Note that as there is no central repository for
service names (they are defined locaaly for each
host), a service name is NOT an appropriate way to
specify a port number for a hypertext address. If the
port number is omitted the preceding colon must also
be omitted. In this case, port number 2784 is assumed
[This may change!].
See also: WWW addressing in general , HTTP protocol .
_________________________________________________________________
Tim BL
W3 ADDRESSES OF FILES
The format of a hypertext reference to a file is an extension of the unix
naming system. The full explicit format is:
file : // node / directories / name
The actual protocols used by the client depend on the implementation of the
browser and the environment. Typically, the browser will check to see
whether the node is the local node, or a node for which files are available
mounted in some form of distributed file system. If neither of these are
the case, then the browser may try rpc, anonymous FTP or other protocols.
Examples
file://cernvax.cern.ch/usr/lib/WWW/defaut.html
This is a fully qualified file name.
fred.html
This relative name , used within a file, will refer to a file of the same
node and directory as that file, but the name fred.html.
Improvements : Directory access
The final file name should be optional. If the address ends with a '/', the
browser should retrieve the contents of the specified directory and generate
a page of virtual hypertext pointing to its contents. In addition, it could
display an information file contained in that directory, if any is present.
Suggested file names to search for in order : README.html, *README*.html,
README, *README*, *readme*.
HYPERTEXT ADDRESS FOR NET NEWS
The format of a hypertext reference to information in the internet/usenet
news system can take any of the following forms:
news: newsgroup This refers to a list of articles currently available
in the given newsgroup. The newsgroup is a series of
alphanumeric characters and dots.
news:* This refers to a list of valid newsgroups.
news: message_id This refers to a given article explicitly. The
message_id is optionally surrounded by angle brackets,
and must contain an @ sign.
Possible extensions to this are more generous wildcarding for the list of
newsgroups. It takes too long to load the whole list, and it would be more
useful to be able to browse through a set of newsgroups.
There is no way of referring to "unread" articles. Keeping track of this is
the job of the browser.
Examples
news:<12345678@cernvax.cern.ch>
news:12345678@cernvax.cern.ch
These addresses both refer to the same (imaginary!) article by its unique
message-id.
news:comp.sys.next.announce
This refers to a list of articles in the newsgroup comp.sys.next.announce.
The list is, of course, a list of references to article by message-id.
TELNET ADDRESSING
A telnet address is a spcecial case of a W3 address.
When a telnet address is used, information can only be rertrieved using an
interactive telnet session. This has the disadvantage that information
cannot be indexed, searched, etc automatically, nor can it be gatewayed into
other systems. The telnet addressing form is used to allow a pointer to
information systems such as library information systems which have not been
gatewayed into the web properly yet.
The syntax is, with [] indicating optional parts (see full BNF)
telnet : / / [ user @ ] host [ : port ]
There should be no spaces. For example, the following are valid telnet
addresses:
telnet://www@info.cern.ch:23
telnet://www@info.cern.ch
telnet://info.cern.ch
user is the optional name of the user to be used for login.
If the username is omitted, then so must be the "@"
sign. This is equivalent to the argument used with the
-l option on the ucb telnet command. When the username
is omitted, some access servers will prompt for a
username and password.
host This is the name of the server in internet form. A
numeric form (e.g. 128.141.201.74) may be used, by the
domain name form (e.g. info.cern.ch) is preferred.
The host is mandatory.
port This is a numeric port number. If a non-numeric string
is used, it must be a defined service name. Note that
as there is no central repository for service names
(they are defined locaaly for each host), a service
name is NOT an appropriate way to specify a port
number for a hypertext address. If the port number is
omitted the preceding colon must also be omitted. In
this case, port number 23 is assumed.
_________________________________________________________________
Tim BL
GOPHER ADDRESSING
Gopher addresses indicate that the gopher protocol should be used to access
the information. The Gopher protocol is a simple internet protocol similar
to HTTP . It allows the transfer of menus or plain text files. (HTTP
expresses both menus and plain text files as special cases of hypertext
files). See the gopher protocol notes .
The syntax is, with [] indicating optional parts (see BNF )
gopher:// hostname [: port ] [/gtype/ [selector] ] [ ? search ]
There should be no spaces. For example, the following are valid addresses:
gopher://gopher.micro.umn.edu:70
gopher://gopher.micro.umn.edu:70/1/
gopher://gopher.micro.umn.edu:70
The W3 address for a gopher item may be derived from the fields of a gopher
menu line which has the format
host This is the name of the server in internet form. A
numeric form (e.g. 128.141.201.74) may be used, by the
domain name form (e.g. info.cern.ch) is preferred. The
hostname is mandatory.
port This is a numeric port number. If a non-numeric
string is used, it must be a defined service name.
Note that as there is no central repository for
service names (they are defined locaaly for each
host), a service name is NOT an appropriate way to
specify a port number for a hypertext address. If the
port number is omitted the preceding colon must also
be omitted. In this case, port number 70 is assumed.
gtype This is a gopher item type number, a (hopefully
printable!) ASCII character. Currently these types
are all ASCII decimal digit characters. Character "0"
(hex 30) signifies a plain text file. Character "1"
signifies a Menu. Character "7" signifies a
searchable index. Character "8" should not be used in
a W3 address: use telnet addressing instead. In
general W3 terms, the type is the first part of the
path. The rest of the path is the gopher selector
string. The type field is a hint to the client as to
how to represent the anchor, and how to follow it.
selector This is the string to be sent to the gopher server to
identify the information required.
_________________________________________________________________
Tim BL
ESCAPING ILLEGAL CHARACTERS
The W3 address syntax allows a path to contain most printable ASCII
characters, but some are inevitably used for punctuation are excluded. W3
addresses are sometimes used to represent addresses in some other space.
This happens when an HTTP server, for example, uses file names as its
document names, or when addresses from some other protocol (Gopher, WAIS,
etc) are mapped into the W3 web.
In these cases, a convention is normally used to map illegal characters in
these "foreign" names onto the allowed set.
In the case of an HTTP server, any mapping may be used.
A suitable convention is that a percent sign (%) followed by two
hexadecimal digits (0-9 or a-f) stands for the single character with ASCII
hexadecimal code represented by those two digits (Most significant digit
first).
A percent sign itself must therefore be represented by %25, as 25 hex is
the ASCII code for "%".
_________________________________________________________________
Tim BL
W3 ADDRESS SYNTAX: BNF
This is a BNF-like description of the W3 addressing syntax . We use a
vertical line "|" to indicate alternatives, and [brackets] to indicate
optional parts. Spaces are representational only: no spaces are actually
allowed within a W3 address. Single letters stand for single letters. All
words of more than one letter below are entites described elsewhere in the
syntax description. (Entity names are here linked to their definitions,
probably making this unreadable with the line mode browser.)
An absolute address specified in a link is an anchoraddress . The address
which is passed to a server is a docaddress .
anchoraddress docaddress [ # anchor ]
docaddress httpaddress | fileaddress | newsaddress |
telnetaddress | gopheraddress
httpaddress h t t p : / / hostport [ / path ] [ ? search ]
fileaddress f i l e : / / host / path
newsaddress n e w s : groupart
groupart * | group | article
group ialpha [ . group ]
article xalphas @ host
telnetaddress t e l n e t : / / [ user @ ] hostport
gopheraddress g o p h e r : / / hostport [/ gtype [ / selector ]
] [ ? search ]
hostport host [ : port ]
host hostname | hostnumber
hostname ialpha [ . hostname ]
hostnumber digits . digits . digits . digits
port digits
selector path
path void | xalphas [ / path ]
search xalphas [ + search ]
user xalphas
anchor xalphas
gtype xalpha
xalpha alpha | $ | _ | @ | ! | % | ^ | | * | ( | ) | . |
digit
xalphas xalpha [ xalphas ]
ialpha alpha [ xalphas ]
alpha a | b | c | d | e | f | g | h | i | j | k | l | m | n
| o | p | q | r | s | t | u | v | w | x | y | z | A |
B | C | D | E | F | G | H | I | J | K | L | M | N | O
| P | Q | R | S | T | U | V | W | X | Y | Z
digit 0 |1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
digits digit [ digits ]
alphanum alpha | digit
alphanums alphanum [ alphanums ]
void
See also: General description of this syntax, Escaping conventions.
_________________________________________________________________
Tim BL
HTML
The WWW system uses marked-up text to represent a hypertext document for
transmision over the network. The hypertext mark-up language is an SGML
format. This defines the basic syntax used. The particular language, the set
of tags and the rules about their use, and their significance is not part of
the SGML standard. There being no standard on this, we have adopted a set
which seems sensible. We call them HTML -- hypertext markup language. HTML
is not an alternative to SGML, it is a particular format within the SGML
rules (an SGML "DTD"). HTML parsers should ignore tags which they do not
understand, and ignore attributes which they do not understand of tags which
they do understand.
See also:
The tags A list of the tags used in HTML with their
significance.
Example A file containing a variety of tags used for test
purposes.
Default text
Unless otherwise defined by tags, text is transmitted as a stream of lines.
The division of the stream of characters into lines is arbitrary, and only
made in order to allow the text to be passed through systems which can only
handle text with a limited line length. The recommended line length for
transmission is 80 characters. The division into lines has no significance
(except in the case of example sections and PLAINTEXT ) apart from
indicating a word end. Line breaks between tags have no significance.
HTML TAGS
This is a list of tags used in the HTML language. Each tag starts with a
tag opener (a less than sign) and ends with a tag closer (a greater than
sign). Many tags have corresponding closing tags which identical except
for a slash after the tag opener. (For example, the TITLE tag).
Some tags take parameters, called attributes. The attributes are given
after the tag, separated by spaces. Certain attributes have an effect simply
by their presence, others are followed by an equals sign and a value. (See
the Anchor tag, for example). The names of tags and attributes are not case
sensitive: they may be in lower, upper, or mixed case with exactly the same
meaning. (In this document they are generally represented in upper case.)
Currently HTML documents are transmitted without the normal SGML framing
tags, but if these are included parsers will ignore them.
Title
The title of a document is given between title tags:
<TITLE> ... </TITLE>
The text between the opening and the closing tags is a title for the
hypertext node. There should only be one title in any node. It should
identify the content of the node in a fairly wide context, and should
ideally fit on one line.
The title is not strictly part of the text of the document, but is an
attribute of the node. It may not contain anchors, paragraph marks, or
highlighting. the title may be used to identify the node in a history list,
to label the window displaying the node, etc. It is not normally displayed
in the text of a document itself. Contrast titles with headings .
Next ID
This tag takes a single attribute which is the number of the next
document-wide numeric identifier to be allocated (not good SGML). Note that
when modifying a document, old anchor ids should not be reused, as there
may be references stored elsewhere which point to them. This is read and
generated by hypertext editors. Human writers of HTML usually use mnemonic
alpha identifiers. Browser software may ignore this tag. Example of use:
<NEXTID 27>
Base Address
Anchors specify addresses of other documents, in a from relative to the
address of the current document. Normally, the address of a document is
known to the browser because it was used to access the document. However, is
a document is mailed, or is somehow visible with more than one address (for
example, via its filename and also via its library name server catalogue
number), then the browser needs to know the base address in order to
correctly deduce external document addresses.
The format of this tag is not yet specified.
Anchors
The format of an anchor is as follows:
<A NAME=xxx HREF=XXX> ... </A>
The text between the opening tag and the closing tag is either the start or
destination (or both) of a link. Attributes of the anchor tag are as
follows.
HREF If the HREF attribute is present, the anchor is
senstive text: the start of a link. If the reader
selects this text, he should be presented with
another document whose network address is defined by
the value of the HREF attribute . The format of the
network address is specified elsewhere . This allows
for the form HREF=#identifier to refer to another
anchor in the same document. If the anchor is in
another document, the atribute is a relative name ,
relative to the documents address (or specified base
address if any).
NAME The attribute NAME allows the anchor to be the
destination of a link. The value of the parameter is
that part of a hypertext address which follows the
hash sign.
TYPE An attribute TYPE may give the relationship described
by the hyertext link. The type is expressed by a
string for extensibility. Strings for types with
particular semantics will be registered by the W3
team. The default relationship if none other is given
is void.
All attributes are optional, although one of NAME and HREF is necessary for
the anchor to be useful.
IsIndex
This tag informs the reader that the document is an index document. As well
as reading it, the reader may use a keyword search.
Format:
<ISINDEX>
The node may be queried with a keyword search by suffixing the node address
with a question mark, followed by a list of keywords separated by plus
signs. See the network address format.
Plaintext
This tag indicates that all following text is to be taken litterally, up to
the end of the file. Plain text is designed to be represented in the same
way as example XMP text, with fixed width character and significant line
breaks. Format:
<PLAINTEXT>
This tag allows the rest of a file to be read efficiently without parsing.
Its presence is an optimisation. There is no closing tag.
Example sections
These styles allow text of fixed-width characters to be embedded absolutely
as is into the document. The format is:
<LISTING>
...
</LISTING>
The text between these tags is to be portrayed in a fixed width font, so
that any formatting done by character spacing on successive lines will be
maintained. Between the opening and closing tags:
The text may contain any ISO Latin printable characters, including the
tag opener, so long as it does not contain the closing tag in full.
Line boundaries are significant, and are to be interpreted as a move to
the start of a new line.
The ASCII Horizontal Tab (HT) character should be interpreted as the
smallest positive nonzero number of spaces which will leave the
number of characters so far on the line as a multiple of 8. Its use
is not recommended however.
The LISTING tag is portrayed so that at least 132 characters will fit on a
line. The XMP tag is portrayed in a font so that at least 80 characters
will fit on a line but is otherwise identical to LISTING. The examples of
markup are here given using the XMP tag.
Paragraph
This tag indicates a new paragraph. The exact representation of this
(indentation, leading, etc) is not defined here, and may be a function of
other tags, style sheets etc. The format is simply
<P>
(In SGML terms, paragraph elements are transmitted in minimised form).
Headings
Several levels (at least six) of heading are supported. Note that a
hypertext document tends to need less levels of heading than a normal
document whose only structure is given by the nesting of headings. H1 is the
highest level of heading, and is recommened for the start of a hypertext
node. It is suggested that the first heading be one suitable for a reader
who is already browsing in related information, in contrast to the title tag
which should identify the node in a wider context.
<H1>, <H2>, <H3>, <H4>, <H5>, <H6>
These tags are kept as defined in the CERN SGML guide. Their definition is
completely historical, deriving from the AAP tag set. A difference is that
HTML documents allow headings to be terminated by closing tags:
<H2>Second level heading</h2>
Highlighting
The highlighted phrase tags may occur in normal text, and may be nested. For
each opening tag there must follow a corresponding closing tag. NOT
CURRENTLY USED.
<HP1>...</HP1> <HP2>... </HP2> etc.
Glossaries
A glosary (or definition list) is a list of paragraphs each of which has a
short title alongside it. Apart from glossaries, this format is useful for
presenting a set of named elements to the reader. The format is as follows:
<DL>
<DT>Term<DD>definition pagagraph
<DT>Term2<DD>Definition of term2
</DL>
Lists
A list is a sequence of paragraphs, each of which is preceded by a special
mark or sequence number. The format is:
<UL>
<LI> list element
<LI> another list element ...
</LI>
The opening list tag (UL for an unordered list, OL for an ordered one) must
be immediately followed by the first list element. The representation of the
list is not defined here, but a bulleted list for unordered lists, and a
sequence of numbered paraghraphs for an ordered list would be quite
appropriate.
"OL" IS NOT CURRENTLY USED