Browser instead of window manager?

yes:

% startx /usr/bin/google-chrome --kiosk

(or use any other webbrowser)

you could also add this to your .xinitrc / .xsession file:

exec /usr/bin/google-chrome --kiosk

Pyro was a short-lived project to build a compositing X11 window manager using Firefox, such that the browser managed the entire desktop.

Try just put this in your .xinitrc file in your home directory.

exec konqueror

Then restart X. If this doesn't work or if you are using gdm, kdm or xdm as a display manager, you can either shut that off and run startx from the command line or you can create an alternate user and configure their .xinitrc to do the same. Then you can try running startx as that user from a virtual console (Ctrl-Alt + F2). It should open another X session. I noticed when I tried this with Firefox that Firefox wasn't filling the screen even if I used a --geometry option. It seems that Firefox may store its geometry preferences in a session. You may have to deal with this on other programs like Chrome.

Keep in mind that a lot of things that these programs might be expecting things that you'd have in a modern desktop like proxy management, drag and drop or special paste buffer handling.

This is all old hat stuff. I'm sure you have a good purpose for it, but a lot of people probably haven't done this type of stuff since the early 2000s. You used to have to mess with it all the time during the FVWM days (before display managers became popular)

I'd also like to clarify something in your question. The program that you exec like this is not a "window manager". A window manager is a specific program that also gets run by X in this fashion or through a desktop manager and allows you an easy way to run (exec) additional programs, usually decorating them so that you have an interface for closing them. In the early days of X, it would just start an xterm and you would run the command for your window manager from there with an &. So we are making progress. ;-)

Some programs can be run outside of a window manager and still allow you to control their window size or bits of their display. For instance, Google Chrome pseudo decorates its windows. However, I tested Google Chrome and it doesn't have or honor enough standard X options to be a good solution here. For instance, it doesn't seem to handle the --geometry= option (shame on you Google). This is another good reason to use Konqueror or Firefox instead.

Sorry for rambling.

Yes, of course -- you only need a window manager if you want to be able to resize the top-level windows using WM decorations. What you can't do is have two window managers running on the same X display at the same time.