HTML Print question

HTML Print question

Hi,

I've to print an HTML  page to print a table :

the Table has a bold separation. But when I want to print it, bold disappears. I thought about the browser's option "print background and images" (essential to print colored background) but it's the same.Do you have an idea to print those bold line please ?

The style of this border is :
<style>
tr.bordertop td {border-top:2 solid black}
</style>

Thanks !