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The @w3c WebFonts working group now has a fully functional Demo of Incremental Font Transfer which compares IFT to normal font loading with Unicode-range static subsetting. Just click on "next text sample" to see the font being upgraded in real time to support more writing systems and more font variation axes. There is a running total of bytes transferred, you can see that IFT loads much less data.
Both CFF2 and glyf fonts are used in the demo.
For the enthusiastic, who want to go beyond a canned demo: it is all on GitHub
https://github.com/garretrieger/ift-demo
There is a client (in rust, transpiled to wasm) with brotli, harfbuz and woff2 support; the sample texts are in a json file so you can edit them or add your own.
Initial font subset, and the various patches, use a mixed (glyph and table keyed) encoding as described here
https://github.com/w3c/ift-encoder/blob/main/docs/experimental/closure_glyph_segmentation.md
and the implementation is also on GitHub
Oh and for anyone who may be thinking of earlier IFT specifications which required a fancy, intelligent server: nope. All static files now, host them as you want, and nicely cache-freindly.
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