To the extent I've used it so far, VS.NET creates pretty compatible code, but to do things that are in the standards that IE doesn't understand, you have to write it yourself. FrontPage used to work pretty much only with IE, but it is a bit better now, although things still work best with IE.
But in this case, Microsoft detected the Opera browser and purposely sent bad code to it, such as the page having a -30 pixel right margin. The site rendered perfectly when Opera was set to spoof the IE6 headers.
Since MS owned the web site that did that, what legal duty did they breach? They don't have the duty to make sure Opera's software runs right, and if I want to disrupt things on my property to keep my competitor from gaining a foothold, I get to do that.