[demallien]

Take a look at Chromium as an example. Every tab gets it's own process. The GPU gets it's own process. Every plugin object gets its own process. Page loading is done in a separate process. Web workers run in a separate process. Chromium will happily eat up every core your CPU has to offer.

[pcwalton]

If you're running multiple tabs at the same time and actually interacting with both. But most of the time you're only laying out and rendering one page at a time; your browser actually only displays one tab at a time, after all.