Adjust `small` font size normalization

Make the font size for `small` not quite so small. The default value of
`smaller` doesn't scale the base font size down by the same proportion
whatever the base size. IE 6/7/8 end up rendering small text larger than
most modern browsers. Opera can render it slightly smaller than Chrome
and Firefox when the base font size is relatively large.

The previous size of `75%` was a bit too small.