What HarfBuzz doesn't do

HarfBuzz will take a Unicode string, shape it, and give you the information required to lay it out correctly on a single horizontal (or vertical) line using the font provided. That is the extent of HarfBuzz's responsibility.

It is important to note that if you are implementing a complete text-layout engine you may have other responsibilities that HarfBuzz will not help you with. For example:

As a layout-engine implementor, HarfBuzz will help you with the interface between your text and your font, and that's something that you'll need—what you then do with the glyphs that your font returns is up to you.