Replace the stylesheet hack with inline styles.

The purpose of the stylesheet hack was to provide a default without
having to use inline styles on containers.  We can do this much more
neatly by instead just giving the inline styles to a parent container,
leaving users free to customize the children.
A better fix for the font-size 'smaller' problem.

This resolves #991, replacing the earlier temporary patch.  It takes
advantage of the fact that line-height can take the form of a unit-less
integer, in which case it mirrors the font-size, even when it is
something abstract, like 'smaller'.  We can then read the dummy
element's height to learn the effective font-size.