Always show a vertical scroll bar

IE always shows a vertical scroll bar for a web page, regardless of whether or not the page is long enough to require one. This is nice for sites with a fixed-width centred layout and a combination of short and long pages, because it means the horizontal position stays exactly the same on every page. Other browsers (Firefox and Chrome at least) don't do this by default.

Finally I've looked up how do this in these other browsers, and discovered that it's actually quite easy. It's not even a hack! Just a CSS rule ...

html {overflow-y: scroll;}