HTML 5 is the next major version of HTML. It introduces a bunch of new elements that will make pages more semantic. This will make it a lot easier for search engines and screenreaders to navigate our pages, and improve the web experience for everyone. In addition, HTML 5 will also include fancy APIs for drawing graphics on screen, storing data offline, dragging and dropping, and a lot more.
CSS 3 builds upon the principles about styles, selectors and the cascade that we know so well from earlier versions of CSS. It adds loads of new features, including new selectors, pseudo-classes and properties. Using these new features it becomes a lot easier to set up layouts.
I may be a huge dork but this kind of stuff get me stoked.