A white paper has recently been published that outlines development, design and deployment guidelines for hosted HTML applications for Windows Media Center in Windows Vista. Included in this documentation is a specific set of factors that developers must consider in Windows Vista that are new or changed from previous versions of Windows Media Center.
If you plan to migrate forward an existing hosted HTML application, I encourage you to check out this new white paper at the following location:
Development and Compatibility Considerations for HTML Applications in Windows Media Center for Windows Vista - http://msdn2.microsoft.com/en-us/library/aa992048.aspx
Also, if you are developing applications using Windows Media Center, I strongly encourage you to first check out the new Media Center Markup Language (MCML) development platform in Windows Vista before performing a simple port of any existing HTML applications or creating new HTML applications. MCML offers many benefits over HTML for Windows Media Center applications, including access to the same UI and animation system used by Windows Media Center itself and full-fidelity rendering on Xbox 360 Media Center extenders. You can download the Windows Media Center SDK for Windows Vista to learn more.