The problem here is that it depends upon your browser's behaviour. If you have a browser that wants to load the entire source before it starts rendering (lynx was and probably still is a good example of this), then the user will see nothing until the process has entirely finished. This is not a good thing.

The other problem is that browsers will usually get bored and time-out if they don't see any connection on a socket for a long enough period of time, or if the connection remained open for too long. So your browser might get bored and drop the connection after 50% of the processing has been done, even if you have been sending "progress updates".

So while you can have a solution that might work 100% of the time under your favourite or most popular browsers, it's not going to work 100% of the time with all browsers. This is where client-pull comes in handy, just about everything supports (or should support) refresh tags, so you don't have to worry about the user's browser getting bored, not displaying the progress reports, or wandering off to do other things.

Cheers,
Paul