Here is a gratuitous annotation, as an example.

Have a look at the Page Notes section above to see another feature provided by Hypothesis.

This test page shows the Hypothesis sidebar open by default, using what they call the classic theme.

I have used my Hypothesis account in order to create this public page note.

I can also create private notes and I don't think I need to have an account for this, probably stored in localStorage (not sure though).