It's actually a good idea.

Moved to feature requests forum

Good Idea.

Also, css could be compressed using gzip

Something like
< http://www.fiftyfoureleven.com/weblog/web-development/css/the-definitive-css-gzip-method >
or < http://www.catswhocode.com/blog/3-ways-to-compress-css-files-using-php >

I would leave this at least configurable, because I believe many hosts transparently compress the CSS using apache's mod_deflate. Anyway, I don't know which is more efficient / resource friendly between mod_deflate or php.

As I wrote, if we have this CSS proxy in place, we can do a lot of fancy stuff with the output, such as removing comments, spaces, compression...

BTW, I have no precise idea about how to achieve the feature, but it would be even nicer if we can use the same architecture to proxy the javascript files also :)

I don't think something can be done about pictures (merge all the pictures into one big file, and use CSS sprites... sounds too complex for automatic processing to me...)

Well, I hope someone gets motivated to implement this :) If it is generic enough, I am sure it can even be useful to projects others that b2evolution...

even nicer if we can use the same architecture to proxy the javascript files also

Since b2evo version 3, javascript are compressed using minify or something like that. It was a great upgrade.

I don't think something can be done about pictures (merge all the pictures into one big file, and use CSS sprites... sounds too complex for automatic processing to me...)

It could be done for skins:
- online css script generator: http://spritegen.website-performance.org/
- offline css script generator: https://launchpad.net/css-sprite-generator

I will try it for evopress skin.

Oh, OK, thank you for the details, I did not know about these. Anyway, I was more referring to "on-the-fly" type of processing, which would allow more flexibility I believe...