I'm Will Mason, and I manage the developer documentation team for Internet Explorer. We write the documentation for HTML, DHTML, CSS, WebBrowser Control , etc. that you can find in the MSDN Library.
You might have noticed lately that the MSDN site has changed its domain name to MSDN2. Beginning last fall, the MSDN (Microsoft Developer Network) group has been using a new publishing tools and had to keep the content published with the old tools separate from the new. As you have probably figured out, MSDN2 is the site that uses the new tools.
You might also have noticed that the URLs have changed more than just in the domain name. To keep our file names unique, the file names are for the most part now page IDs, rather than page titles. For example, "item" is a property of many objects and there are several "item" topic pages in the Library. The IDs make sure that no two "item" topic pages can have the same URL.
Over the winter, documentation sets were slowly migrated from one domain to the other. At some point in the future, the MSDN2 domain will once again become just MSDN and will keep the new scheme for file names.
So, what does this mean to you? Well, actually not much. If you're creating links today, you will have to use the MSDN2 URLs. Old URLS that point to MSDN (the old domain name) are redirected to MSDN2, so you don't have to worry about them.
If you link to MSDN2 pages, redirects will be put in place when MSDN2 reverts to MSDN. These redirects will not go away. Over time, you might want to correct them, but they will always work.
For more on the reasons behind these changes, please see http://msdn.microsoft.com/msdnmag/issues/05/02/InsideMSDN/default.aspx. Keep watching the http://msdn2.microsoft.com for more information.