How does navigator remember place on page?

[deepthings]

I have seen some sites where you will be reading a page, you scroll down, then link away to a different page in the same window/tab, and upon your return, the first page remembers what part of it you were reading before, automatically focussing there. It makes browsing much smoother.

An example: http://www.guardian.co.uk/environment/2008/mar/25/biofuels.energy1

How is this done?

Thanks,
Matt