Code validation

You can make sure that your code is correct by having it validated. There are various sites that will do this for free.

Why validate:

Many browsers will accept incorrect code, and guess a way to interpret it. Different browsers might reach different conclusions.

Moreover, at some point in the future it could happen that your code is not recognized anymore, or produces an outcome different from what you intended.
[On the other hand, browsers will - in general - understand correct code from a previous version.]

How to do it:

In general, you add at the beginning of your HTML file (before any HTML code) the lines

<?xml version="1.0" encoding="iso-8859-1"?> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

and then submit it to a validation site.

Here are the details.

You have to describe the encoding used (some validating sites guess it, or give you a choice).
One way is to do it before the HTML code. The next line takes care of the common choice of characters, ISO 8859-1, (Latin-1):
<?xml version="1.0" encoding="iso-8859-1"?>

Alternatively, add in the <head>...</head> part of the HTML code the line (note the use of quotation marks - it is not a typo)
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
In order to validate your code, you have to declare the standard to which it adheres.
To describe the HTML standard (the document type declaration, DTD), the file should contain a DOCTYPE declaration (before the HTML code).
Here are a few examples (from http://www.htmlhelp.com/tools/validator/doctype.html).
The information about the URL against which to check the file need not be provided.
- HTML 4.01 Transitional:
  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
  or
  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
  "http://www.w3.org/TR/html4/loose.dtd">
- HTML 4.1 Frameset:
  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN"
  "http://www.w3.org/TR/html4/frameset.dtd">
- HTML 4.0 Strict:
  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN"
  "http://www.w3.org/TR/REC-html40/strict.dtd">
- HTML 3.2:
  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//ENC">
Then, you submit your file to a validator (either on the Internet, or on your machine.)

Where to validate:

The Web Design Group's validator is at http://www.htmlhelp.com/tools/.
It can validate HTML and CSS1 (Cascading Style Sheets, version 1).
The CSS validator does not seem to like CSS included in an HTML file.
Help is also available.

The World Wide Web Consortium's validation site is http://validator.w3.org/.
This site offers validation of Cascading Style Sheets (CSS) as well.
To see what it does (and why), look at http://validator.w3.org/docs/help.html.