After finally reaching the stage where I have accumalted enough programming knowledge ( hopefully ) to re-design my site using CSS3, I am left with only 1 challenge: Xhtml 1 or html 5.
From all I have read, I think I understand that xhtml gives better options to optimise for Android/Iphone/small screen platforms, but can have some compatibility problems for various browsers/systems.

Is there a concrete downside to using Xhtml? If so, what is it?