HTML References:

Warning: treat the information below with a grain of salt, as new features are being still added, others considered deprecated (therefore, might not be supported by future browsers). As of 2005, the current HTML version is 4.*, which is being incorporated into XHTML (Extensible HTML). The previous version is 3.2.
Details provided by the World Wide Web Consortium.

Here are a few links that offer more details (expected to be correct):

What is a MARKUP language?

HTML (HyperText Markup Language) is a "markup" language: the regular text is enhanced with information about its content.
The HTML commands instruct the browser which displays the document how to present it. The idea is that you specify the LOGICAL description of what you want realized (e.g., bold text, heading, horizontal rule, new paragraph), and let the browser do it (i.e., the PHYSICAL realization) using its own means.
Thus, different browsers can "execute" differently the commands.

Basic syntax:

In older HTML versions the commands and attributes were not case-sensitive, but in view of the new standards, use lower case. Almost all commands are of the form
<tag ATTRIBUTES> content </tag>
For example, a link to a JPG image:
<a href="../image.jpg" alt="photo" width="70">total moon eclipse</a>

Except with a few cases (e.g., <br /> for LINE-BREAK, <hr /> for HORIZONTAL-RULE), the HTML 4.* syntax requires the end-tag as well.
Browsers might not complain about the end-tag being omitted in some cases. E.g., PARAGRAPH, <p> was accepted without its end tag, </p>. However, for compatibility with newer versions, add the end tag as well.

The attributes are often optional.
To conform with the new standards, all attribute values must be quoted. HTML 4 required quotes only if the attribute value contained any character other than letters (A-Za-z), digits, hyphens, and periods.

Comments are enclosed between <!-- and -->.

Some of the tags (and their end-tags) are required: <html>, <head>, <title> and <body>.

An example:

Below is the content of a web-page (click here to see it displayed): 



<?xml version="1.0" encoding="iso-8859-1"?>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd"> 

<html>

<head>
  <title>
    HTML: Web-page example
  </title>
</head>

<!-- comments have to be enclosed exactly like this -->
<!--  note that the "title" tag is part of "head" -->

<!-- 

the HTML below is not always correct, it is meant to illustrate the main
ideas

 --> 

<body>

<p>
  There are various fonts available: <b>bold</b>, <i>italic</i>,
  <tt>teletype</tt>. You can also <em>emphasize</em> the text, and use
  HEADINGS (which will start on a new line), from

  <h1>level 1</h1> to <h6>level 6</h6>
</p>

<p>
  Note that extra spaces            and 

  line-breaks are not displayed, unless you request "PREFORMATED"
  text, as below:

  <pre>
first line                big space
next line
  </pre>
</p>

To break a line use <br /> and start a new paragraph with

<p>(this gives more vertical space).</p>

<p>
  One important feature of HTML is that we can refer to
  files <b>anywhere</b> on the Internet, using LINKS. E.g.,
  this <a href="http://www.math.uh.edu">first</a> link takes you to the
  home-page of the Math Dept,
  the <a href="introduction.html#TheEnd">second</a> link takes you to the
  bottom of the file from which we got here, and
  the <a href="#bottom">third</a> link takes you to the bottom of this page
  (notice how that location was defined!). Use the <b>"BACK"</b>
  button on your browser to return here!

  <br />

  The location of a file is given by its URL (Uniform Resource Locator; a
  more general term is URI, Uniform Resource Identifier). An URL can be:
  <blockquote>
	
    ABSOLUTE, as "http://www.math.uh.edu", or

    <br />

    RELATIVE, as "../web.html", which points to the file
    <tt>web.html</tt> in the directory above the file currently displayed.

  </blockquote>

  An URL describes also the way to handle the file,
  e.g. <a href="mailto:SomeBody@math.SomeWhere.edu">send e-mail</a>,
  or <a href="ftp://ftp.gnu.org/pub/gnu/GNUinfo">process an ftp
  request</a>.
</p>

<br /> <br /> <br /> <br /> <br />

<strong>Here are a few often used commands:</strong>

<p>
  A HORIZONTAL RULE is produced with
  <hr />

  Here is an ORDERED LIST:

  <ol>
    <li>first List Item</li>
    <li> the second item is an UNORDERED LIST:
      <ul>
	<li>one item</li>
	<li>another item</li>
      </ul>
    </li>
    
    <li> we can also use DEFINITION LISTS:
      <dl>
	<dt>Definition Term</dt>
	<dd>Definition Description</dd>
      </dl>
    </li>
    <li>fourth item</li>
  </ol>
</p>

<p>
  Include a picture that changes size with the window (try to resize the
  window)

  <img src="http://www.math.uh.edu/uhbanner.gif" alt="UH banner"
       width="40%" />

  <br />

  or stays of fixed size (we also added a border, and made it stay in the
  center of the line, aligned at middle to the text)
</p>

<p align="center">
  a <img src="http://www.math.uh.edu/uhbanner.gif" alt="UH banner"
	 align="middle" border="5" height="47" /> a
</p>

We can even make a link out of this picture (notice the border colored as a
link, as long as we do not forbid it with <code>border="0"</code>).

<a href="http://www.uh.edu"><img src="http://www.math.uh.edu/uhbanner.gif"
alt="UH banner" height="20" /></a>

<br /><br />

<a name="bottom">BOTTOM</a>

</body>
</html>
(the bottom)