If you want to serve an empty, broken page as a first impression, you can. I strongly recommend against it because it makes your site look half-finished, poorly designed, and generally unprofessional.
> available in the browser
That isn't always true. You don't know anything about the browser a priori.
> make a better experience
Of course. Javascript can add nice features, which is why you progressively enhance the page to add features when the underlying requirements are available. Do you skip the check for NULL when calling fopen(1)? Checking for errors is always important. On the web, this means you should gracefully handle problems like missing javascript (intentionally or otherwise) or other resources.
> Making UIs respond faster
Requiring two additional HTTP requests (one to load javascript, and another to load the actual page content) isn't making the page faster. After the first request your javascript will be available most of the time for partial loads in any case.
> interactive content
That is rarely a good idea, but it's your page so do what you want. Progressive enhancement doesn't affect this at all.