I cannot see any tables with rounded corners on that site, what particular ones are you refering too ?
You can do "view source" on any web page to see what HTML is being used.
The BBC are using CSS (Cascading Style Sheets) so maybe they are doing something fancy with that.
Once you find out the name of the CSS file they are using it is easy to download it to your PC and study it. Just put the name of the CSS file after the web page address and it will ask you if you want to download it.