HTML files are text files featuring semantically tagged elements.
HTML filenames are suffixed with .htm or .html.
Here's a simple HTML file:
<html>
<head>
<title>My First HTML Document</title>
</head>
<body>
<h1>A level one heading</h1>
Hello there. This is <STRONG>very</STRONG> important.
</body>
</html>
Elements that are enclosed in angle brackets like <html>,
<head>, and <title> are called tags. Tags are case
insensitive. <html> means the same thing as <HTML> as <Html> <HtMl>.
Most tags are matched with closing tags, and affect the text contained between
them. The closing tag is the same as the opening tag except
for a / after the opening angle bracket. For example, </html>,
</head>, and </title> are closing tags. The text in between
<title> and </title>, My First HTML Document in the above example,
is the title of the page.
As you can see from the above example tags may, in general, nest. However they may not overlap (though some browsers can handle this).
Some tags have attributes. An attribute is
a name, followed by an = sign, followed by the value. For example, to make a centered H1 heading, use the
ALIGN attribute with value center; i.e.
<h1 align="center">A level one heading</h1>
For more information about HTML see Larry Aronson and Joseph Lowry's The HTML 3.2 Manual of Style (Ziff-Davis Press, 1997) or the NCSA Beginner's Guide to HTML.