Specifying Tab-Width?

Use the tab-size property. You’ll need vendor prefixes currently. Example:

pre
{
    -moz-tab-size: 4;
    -o-tab-size:   4;
    tab-size:      4;
}

See also the article on developer.mozilla.org: tab-size.

.tabstop
{
    -moz-tab-size: 4;
    -o-tab-size:   4;
    tab-size:      4;
}

Unstyled tabs (browser default)
<pre>
	one tab
		two tabs
			three tabs
</pre>

Styled tabs (4em)
<pre class="tabstop">
	one tab
		two tabs
			three tabs
</pre>

I believe this blog post should help you out:

Here's a solution, it's not neat since it has to be done for every instance of a tab, but it makes the tabs take up less space and preserves the formatting for copying out of the browser (obviously replace "A SINGLE TAB HERE" with a real tab, this blog software automatically removes tabs from entries it seems):

<span style="display:none">A SINGLE TAB HERE</span><span style="margin-left:YOUR NEW TAB WIDTH"></span>

Basically, replace every instance of a tab in your code with that code snippet (after choosing a suitable width, you could do it in a stylesheet pretty easily). The code artificially inserts the margin whilst keeping the original tab in the code ready for copy/pasting.

Incidentally, it looks like tab stops made it into the CSS specification.

There's also another Stack Overflow question on this subject.

As George Stocker pointed out tab stops should be coming along in a future CSS (FF4 should have it), but in the mean time...

The problem with the linked blog post is that the tabs aren't copied when copying/pasting from the browser. As an alternative try the following:

<style>
.tabspan{
    display:inline:block;
    width:4ex;
}
</style>
<pre>
int main()
{
<span class=tabspan>\t</span>return 0;
}
</pre>

Where "\t" in the above is the actual tab character. Now it should copy and paste properly. Not as nice as slapping a css property on the <pre> tag, but such is life.

(P.S. answered this old post as its high on google for 'css tab width' and I came up with this solution shortly after coming here.)