You asked whether HTML is allowed within Python, which indicates that you still think too much in PHP terms about it. Contrary to PHP, Python was not designed to create dynamic web-pages. Instead, it was designed as a stand-alone, general-purpose programming language. Therefore you will not be able to put HTML into Python. There are some templating libraries which allow you to go the other way around, somewhat, but that s a completely different issue.
With things like Django or TurboGears or all the other web-frameworks, you essentially set up a small, stand-alone web-server (which comes bundled with the framework so you don t have to do anything), tell the server which function should handle what URL and then write those functions. In the simplest case, each URL you specify has its own function.
That handler function (or view function in Django terminology) receives a request object in which interesting info about the just-received request is contained. It then does whatever processing is required (a DB query for example). Finally, it produces some output, which is returned to the client. A typical way to get the output is to have some data passed to a template where it is rendered together with some HTML.
So, the HTML is separated in a template (in the typical case) and is not in the Python code.
About Python 3: I think you will find that the vast majority of all Python development going on in the world is still with Python 2.*. As others have pointed out here, Python 3 is just coming out, most of the good stuff is not available for it yet, and you shouldn t be bothered about that.
My advise: Grab yourself Python 2.6 and Django 1.1 and dive in. It s fun.