CSS way to horizontally align table

Try this:

<table width="200" style="margin-left:auto;margin-right:auto">

Simple. IE6 and above will happily center your table with "margin: 0 auto;" if only the page renders in "standards" mode. To make this happen you need a valid doctype declaration, such as

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

or

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

True, IE5.5 and below will still refuse to center the table but perhaps you can live with that, especially if the page is still functional with the table left aligned. I think by now users of IE5.5 and below are fairly used to some odd looking websites - but you still need to ensure that those visual glitches don't render your site unusable.

Happy coding!

EDIT: Sorry, I should perhaps point out that you do not have to have a "strict" doctype to get IE6 and up into "standards" rendering mode. I realised it might seem that way from the doctype examples I posted above. For example, this doctype declaration will of course work equally:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

Steven is right, in theory:

the “correct” way to center a table using CSS. Conforming browsers ought to center tables if the left and right margins are equal. The simplest way to accomplish this is to set the left and right margins to “auto.” Thus, one might write in a style sheet:

table
{ 
    margin-left: auto;
    margin-right: auto;
}

But the article mentioned in the beginning of this answer gives you all the other way to center a table.

An elegant css cross-browser solution: This works in both MSIE 6 (Quirks and Standards), Mozilla, Opera and even Netscape 4.x without setting any explicit widths:

div.centered 
{
    text-align: center;
}

div.centered table 
{
    margin: 0 auto; 
    text-align: left;
}


<div class="centered">
    <table>
    …
    </table>
</div>

Although it might be heresy in today's world - in the past you would do the following non-css code. This works in everything up to and including today's browsers but - as I have said - it is heresy in today's world:

<center>
<table>
   ...
</table>
</center>

What you need is some way to tell that you want to center a table and the person is using an older browser. Then insert the "<center>" commands around the table. Otherwise - use css.

Surprisingly - if you want to center everything in the BODY area - you just can use the standard

text-align: center;

css command and in IE8 (at least) it will center everything on the page including tables.