What is HTML

HyperText Markup Language (HTML) is a language to specify the structure of documents for retrieval across the Internet using browser programs of the WorldWideWeb.

HTML is an application of the Standard Generalized Markup Language (SGML) which is the International Standard (ISO 8879) for text markup. The principle is that text markup concentrates on structure rather than appearance, making the files more reuseable and leaving the visual details to the end-user software (like the browser you're reading this with now). For the reasons why, see Eliot Kimber's comments.

Details of the specification are in the IETF Draft and the HTML Document Type Description. There is a FAQ (Frequently-Asked Questions) document (also available by anonymous ftp from rtfm.mit.edu), and a new book on HTML and the WorldWideWeb out shortly.

Simon Spero (ses@tipper.oit.unc.edu) explained it as:

The HTML DTD with its very simple element structure is primarily intended for describing the structural elements that appear on hypertext pages. Not the structure of the documents that comprise those pages (it's too vanilla for that), but the pages themselves. Not the layout of the pages, but the structure.

In a hypertext browsing system, the page is the basic object into which elements are placed, and which is common to all documents across all display technologies. Much of the structure of a document might be implicitly expressed via links between pages.

In order for a page to be displayed and browsed through correctly on a variety of systems - one of the primary design goals for the web - the layout of a page must be described in a sufficiently abstract way as to make sense on a VT100, a NeXT, or an X workstation. This rule was broken somewhat by the <img> tag, introduced by NCSA for their Xmosaic browser, but is otherwise generally intact. Attributes like centering are only really suitable for bitmapped displays with variable spacing. To allow portable display it is much better to indicate the visual role to be played by the attribute (the reason why you wanted it centered), and allow the display engine to decide how that text should be rendered.

The Universal Resource Locator

The Universal Resource Locator (URL) is the `address' of a resource in the Web. It could be a file, or an index, or some program that does processing: they all use the same format to refer to them by:

scheme://host[:port]/path/filename{#location|?indexterm}

Examples

ftp://www.ucc.ie/pub/sgml/p2sg.ps

http://www.ucc.ie/cgi-bin/acronym?url

gopher://ds.internic.net/

where
• scheme is one of http, ftp, gopher, wais or file, depending on the kind of service you want to access
• [square brackets] mean the :port is optional (the default port is the one for the scheme being used, so leave it out unless told otherwise)
• host is the Internet hostname of the machine where the server runs which provides the resource you are looking for, eg www.ucc.ie
• path is the directory path to the resource
• filename.type is the filename (including the filetype, if needed)
• location is the identity of a known location in a .html file being retrieved (eg you know it has already been marked in the file by the author using the name attribute to an anchor element
• indexterm is a search term or argument to pass to the resource. This is only meaningful if you know the resource is a script or a program which can handle data input

There are two exceptions to this format, for Usenet news and for Telnet.

Telnet

The double slash between the scheme and the hostname is omitted. An optional UserID can be included before the hostname, separated by a colon and an `at' sign, to cause the browser to prompt the user to login with this UserID. There is no path, filename, location or indexterm, eg:

telnet://library:@iruccvax.ucc.ie

Usenet news

The double slash between the scheme and the hostname is omitted, and the hostname is also omitted, as it will default to whatever your browser's configuration points at, so the newsgroup name follows immediately after the scheme. It is possible to follow the newsgroup name with a hash mark and location, the value of the location being the news article ID, but this is not recommended, as most sites expire articles after a certain time, so the chances of an article persisting are very remote:

news:comp.infosystems.www.users