Generally on-the-fly by the web server, although you can, in theory, zip manually and serve. The big speed bottleneck when serving pages is the network so the

Generally on-the-fly by the web server, although you can, in theory, zip manually and serve. The big speed bottleneck when serving pages is the network so the smaller the file you send across the wire the faster your page will load. This goes for all the text mime-types: html, xml, css, javascript, text.