Websites don't need to be pixel-perfect
A few years ago I got a book on web design out of the library on a whim. While it didn't have much new information for me, it mainly seemed to be concerned with pushing the benefits of CSS, it did provide an interesting insight into the history of web design.
But perhaps the biggest benefit I got from it was the idea that web sites don't have to look pixel-perfectly the same in every browser. Despite the huge advances made in web standards with all the different browsers, rendering engines, OSes and the fact that IE6 is still clinging on for dear life means that getting a pixel-perfect match on every browser is near impossible.
Of course you want to achieve the same look and feel across all browsers, but a look and feel is too complex and subtle to simply be destroyed by the fact that a border is a pixel wider in IE. To demonstrate, see if you can guess which websites these pictures are supposed to be mimicking.
Despite the fact the size, colour and some of the layout are wrong you should still be able to recognise at least the first one. If your design requires a certain border thickness or a div to be an exact size and hue in order to be recognisable then your branding isn’t strong enough — back to the drawing board.
Of course the site should be recognisable, and above all usable in every browser you are aiming to support. However, design shouldn’t be at the cost of clarity of the source code and maintainability.
Your content should be visible, your pages navigateable and look appealing, but don’t fret too much if Opera doesn’t show your pretty rounded corners.