If the ruby text is wider than the base text you can specify whether or not it overhangs any surrounding base text, and to what amount, using a property for ruby overhang.

The example on the slide shows the effect of setting a ruby-overhang property to start. Note how the ruby text overlaps the preceding characters, but not the following base text. Note also that ruby does not overlap itself.

For a more detailed description of the currently proposed approach to ruby styling see Ruby Markup and Styling.