A specimen book is meant to answer one question: what does this face look like when it is set as text, rather than as its own name at 72 points. Most online specimens answer a different question, because they load the face over the network and show it in a vacuum.
These pages do neither. Every face here is one your operating system already ships, requested through the same stack a real stylesheet would use. Nothing is downloaded. If a face is missing on your machine you will see the fallback, which is itself the useful information — that is exactly what your readers would get.