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@ >
To: Mark Alexander Davis-Craig <>
Subject: Re: Is there a paper which describes the www protocol?
Cc: www-talk

> From: Mark Alexander Davis-Craig <>

> 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  
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.bitnet        (313)-936-2110

Tim Berners-Lee             
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

    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 .


   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.


   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 .


   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

    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.


   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

    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

    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 .



   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


   This refers to anchor "greg" in the same document as that in which the name

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

  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

   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

    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".



   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 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,

  hostname                This is the name of the server in internet form. A
                         numeric form (e.g. may be used, by the
                         domain name form (e.g. 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.



   This is a fully qualified file name.


   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.



   These addresses both refer to the same (imaginary!) article by its unique

   This refers to a list of articles in the newsgroup
   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




  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. may be used, by the
                         domain name form (e.g. 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:




   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. may be used, by the
                         domain name form (e.g. 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

    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 | $ | _ | @ | ! | % | ^ | | * |  (  |  ) | . |

  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 ]


  See also: General description of this syntax, Escaping conventions.


                                                                         Tim BL


   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

  Example                 A file containing a variety of tags used for test

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.


   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.


   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

  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.


   This tag informs the reader that the document is an index document. As well
   as reading it, the reader may use a keyword search.



   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.


   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:


   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:




   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.


   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


   (In SGML terms, paragraph elements are transmitted in minimised form).


   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>


   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

        <HP1>...</HP1>   <HP2>... </HP2> etc.


   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:


        <DT>Term<DD>definition pagagraph

        <DT>Term2<DD>Definition of term2



   A list is a sequence of paragraphs, each of which is preceded by a special
   mark or sequence number. The format is:


        <LI> list element

        <LI> another list element ...


   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