We no longer maintain a repository of demos.
We have some examples from a 2008 Google tech talk showing some HTML demos written on the fly. Some of the demos, as well as the video from the talk, are available online.
Other presentations are listed in our wiki.
These demos were created to show some of the features of Web Forms 2. Not all these features were kept, but the demos remain for those looking at the history of HTML.
datetimeinput type of Web Forms 2.0
formattribute in Web Forms 2.0