The site is in the form of a Gernsback machine, a hypernavigable multidimensional facet tree for which the languages, systems, organisations and people are instantiations, and which permits exploration by stepping, flipping, and zooming in and out on any of the points of data. For more information on the theoretical basis of the system, you can read the "Museums and the Web" paper here.
The key to site hyperlink styles:
Information page a page that contains information about a language, machine, person or organisation
Pop-up these appear on the information pages only, and enable stored examples, text extracts, images, to pop out in a small window
Step «‹← →›» Moves you to the next or previous value for the current aspect. If in an information page it can jump to next/previous letter, or first/last record as well
Zoom↑↓↔ Zooms out to a listing page (ie generalise) for the selected value, or in to a listing page (ie constrain) for that value. Above a certain level, a summary page will be shown instead
Flip⊕ Flips you around to a nexus page with a list of peer-languages for that parameter, with a set of steps and zooms appropriate to that facet
External link Opens up the stored link in a new window, as it is outside the HOPL site. NB The material stored in these pages is not under the control of HOPL, so can't be guaranteed to be either accurate or polite