The last one in the list, "ZinZin", has "Swash" that makes discrete changes to different characters. And "Lab" also has four very different parameters: "Bevel", "Oval", "Quad" and "Size". "Devovar" has a LOT of weird parameters!
Aha, you can click on any of the checkboxes next to the parameter names to map them to the two-dimensional variable options puck. It helps to break the text sample into several lines and zoom the font size way up to appreciate it.
Using variable fonts instead of color for syntax highlighting would make code look less like puking up a roll of skittles. Especially with some variable fonts designed to express the syntax and semantics of the language. Parens could get huggier as the got deeper!
I'd love a handwriting font with a continuously variable parameter that drove something like perlin noise to make each character slightly different.
Reminds me of the weird property names like "BlueFuzz" that Adobe Type 1 fonts used.
>The story I heard was that Adobe's Type 1 font encoding  used obscure names like BlueValues, OtherBlues, FamilyBlues, FamilyOtherBlues, BlueScale, BlueShift, and BlueFuzz, so that Adobe employees could discuss their proprietary font hinting algorithms in public while they were in line for burritos at La Costeña . That way nobody from Apple or Sun or SGI who was standing in line next to them could understand what they were talking about.
Yes THAT World Famous La Costeña: Guiness Record Holder for the World's Largest Burrito!  On May 3rd, 1997 La Costeña of Mountain View, California created the world's largest burrito. The burrito weighed in at 4,456.3 pounds and was measured at 3,578 feet long. It was created at Rengstorff Park in Mountain View.
I won’t offer suggestions on fixing that, because the straightforward solutions do damage in other ways; a more carefully thought-out solution will be required.
(Firefox Nightly, Windows.)
(You can experimentally enable it in Firefox 61 by setting layout.css.font-variations.enabled to true in about:config.)
I noticed only a few fonts allowed slant. Does this depend on the TTF/OTF we use?
At least FF Meta has an italic axis, but only two discrete values for it, which comes out at more or less the same as two different fonts.
Felt like the font catalog was small. Some back info about what fonts this can play with, and why, and how to add more would help
(not a typography or font expert.)