Web pages are stored as standard ASCII (American Standard Code for Information Interchange) files. Web pages may be viewed by a variety of different Web browsing tools, each of which may have different abilities. However, since Web pages are text files, each Web browser can read it and format the document in accordance with its abilities.
HTML is a standard which enables you to request a Web browser to format and display your Web page in a particular way. HTML allows you to mark areas of your document that will become for example: titles, new paragraphs or italic text. Since the Web page is specified as an ASCII file the codes or "elements" (as they are known) have also to be ASCII.
The elements can broadly be divided into two main categories:
- Those that describe the format of the Web document, i.e. what it looks like,
- Those that define information about the document, i.e. its title
HTML Elements
HTML elements are the mark-up codes in the Web page which instruct the Web browser how the Web page should be formatted when viewed. Elements are denoted by the following: <element_name>. An element tag is the element name surrounded by angled brackets. Elements can either be:
Standard Elements
Standard elements are those which mark a particular section of the document for formatting. The start of the section is indicated with a tag such as: <element_name> and the end of the section with a corresponding tag: </element_name> (note the / symbol).
An example of a standard element in HTML is the heading element. The following example displays the text as a heading size one (this is a large heading):
<H1>This is a Level One Heading!</H1>
Empty Elements
Some HTML elements do not have corresponding end tags. These elements only effect the document at the point which they are placed and do not effect a section of the document as the standard elements do. Examples of empty elements include <P> which forces the follows text to start a new paragraph and <HR> which draws a horizontal line across the page. Examples of the use of empty elements is shown below:
This text is above a horizontal line
<HR>
This text is below the line
<P>
This text will form a new paragraph
More on Elements
Element names are case independent. Any of the following are valid element names: <P>, <p>, <HR>, <hr> etc. Some elements can have arguments. Element arguments pass parameters to the program which is interpreting the HTML. For example, HTML Web documents can include graphics. Since the Web document is a simple ASCII file and not all Web browsers support graphics, the image is held separately. The HTML element which indicates an image should be included has an argument which tells the browser where the image can be found, i.e.:
<IMG SRC="http://osiris.sunderland.ac.uk/sst/sign2.gif">
The above element <IMG SRC> has the argument: "http://osiris.sunderland.ac.uk/sst/sign2.gif", which tells the browser the name and location of the graphic file.
The Structure of an HTML Document
AN HTML document is divided into two main sections: the HEADing and the BODY. The HEADing contains information describing the document, such as its title, etc. The BODY section is where the Web document text etc. is placed. The heading section is indicated by use of the elements <HEAD> and </HEAD> and the body section by the elements <BODY> and </BODY>. The HEADing section should be placed before the BODY. Surrounding these is a single element called <HTML> .. </HTML> which indicates that the following is an HTML document.
The following is an example of an HTML document with nothing in the HEADing section and some text in the BODY:
<HTML>
<HEAD>
</HEAD>
<BODY>
This is some text which would appear on the page!
</BODY>
</HTML>
Most HTML Browsers do not require that the elements: <HTML> <BODY> or <HEAD> (and their corresponding end elements) are included in a Web document. However, it is good practice to include them.
Naming Conventions
The following are various naming conventions for HTML and associated file types.
HTML Documents
HTML documents were first commonly available via Web browsers that ran on UNIX systems. The convention was that an individual Web page would be identified by the file name: name.html, where name is the name of the document and html indicates that it is an HTML document.
However, while UNIX machines place no restriction on the length of filenames DOS computers have (among others) a limit of three characters in their extension. Also on DOS based computers, filenames are stored as UPPER-CASE character file names. Thus, HTML filenames such as NAME.HTM are now not uncommon.
Graphics Files
HTML supports two main graphics file formats:
- GIF - Graphics Interchange Format (CompuServe, pronounced JIF)
- JPEG - A jpeg encoded graphics file (pronounced JAY-PEG)
It is common practice that the file names for GIF graphics images to end with the extension .gif and JPEG images to have the extension .jpeg or .jpg.
Other files
The following are other commonly used file extensions for different file types:
- .txt - An ASCII text file
- .exe - An executable DOS program
- .zip - A compressed file requiring PKUNZIP to decompress, usually on a DOS system
- .Z - A compressed file, usually on a UNIX system
Conforming to these standards helps to avoid confusion.
