I need to redesign the old site and was thinking what would be the best Web markup language to use. Here are my two questions. Please help me to find the right answer to both or any of them:

1. What's the optimal Web standard for desktop users? HTML? XHTML? Anything else? What version of it (e.g. "XHTML 1.0 Transitional")?
2. What is the optimal markup language for mobile users? The site I'm going to redesign is about applications for mobile devices. So, naturally I have a lot of mobile visitors. Most of the devices are Android-based, iOS-based as well as some BlackBerries, Palm OS and Symbian devices. Which markup language should I stick to when I'll be developing mobile version of the site?

Thanks as always for the great answers!