Html or xhtml for newbie

I am reading the excellent Head First HTML book. It's a little bit outdated because it predicted that there would not be a HTML5 because XHTML would become the standard. Now it seems with HTML5 this is not correct and there are going to be both HTML and XHTML standards from now on. For some exploring HTML and CSS and later hopefully Javascript with a view to making some apps for mobile devices which is the standard to choose?