You can use more than one external CSS sheet for a single HTML page?

I wanted special scrollbars, but the CSS for that wouldn't validate, and I wanted my setup CSS to be valid. So, today, I tried an experiment: Put all the scrollbar stuff in one CSS sheet, seperate from the setup sheet, and called it scrollbar.css. I then had a link to BOTH sheets (i.e. two <link>s) in my head, and lo, and behold, I had my scrollbars.

yes, you are correct. a number of style sheets may be applied to a single page.

for validating scrollbars, this may be used...
<script type="text/javascript">
window.onload = function()
{
var htmlStyle = document.getElementsByTagName('html')[0].style;
var bodyStyle = document.body.style;
bodyStyle.scrollbarFaceColor = htmlStyle.scrollbarFaceColor = '#000000';
bodyStyle.scrollbarArrowColor = htmlStyle.scrollbarArrowColor = '#000000';
bodyStyle.scrollbarTrackColor = htmlStyle.scrollbarTrackColor = '#000000';
bodyStyle.scrollbarShadowColor = htmlStyle.scrollbarShadowColor = '#000000';
bodyStyle.scrollbarHighlightColor = htmlStyle.scrollbarHighlightColor = '#000000';
bodyStyle.scrollbar3dlightColor = htmlStyle.scrollbar3dlightColor = '#000000';
bodyStyle.scrollbarDarkshadowColor = htmlStyle.scrollbarDarkshadowColor = '#000000';
}
</script>

edit... there should be no space in this part... ('htm l')

The above example technically validates, but it is flawed if it is misused. The JavaScript is simply skipped over by validators. For example: if you used JavaScript to put a <font> tag into your page, the code would validate at W3C but on screen the outputted document wouldn't actually be valid - which defeats the point of wanting to validate in the first place.

A different approach would be to use IE conditional comments:
<style type="text/css"> @import url('my_style.css'); </style>
<!--[if IE]>
<style type="text/css"> @import url('my_scrollbars.css'); </style>
<![endif]-->
The W3C CSS validator should ignore the stylesheet and not attempt to validate it - but I haven't checked this because I think IE is two piles of pants.

I much prefer the import method of calling in multiple stylesheets. I tend to use <link>'ed stylesheets only when I want the user to be able to switch between different styles (alternate stylesheets).