Docs are available here: https://developers.google.com/fonts/docs/css2

Changes:

• different syntax for multiple font families - https://developers.google.com/fonts/docs/css2#multiple_families
• different format and style syntax - https://developers.google.com/fonts/docs/css2#axis_ranges
• "As a general note, the updated CSS API is more strict about what requests are accepted than the original CSS API." - https://developers.google.com/fonts/docs/css2#strictness
• a display parameter is supported - https://developers.google.com/fonts/docs/css2#use_font-display
• a text parameter is supported - https://developers.google.com/fonts/docs/css2#optimizing_your_font_requests
• the subset parameter is no longer supported
  • for modern browsers, a font-face with a unicorn-range prop (https://developer.mozilla.org/en-US/docs/Web/CSS/%40font-face/unicode-range) is returned for each available subset
  • for legacy browsers (https://caniuse.com/#feat=font-unicode-range), a font with all subsets is returned
  • some info: v2 API for old browsers google/fonts#2379

Things to do:

• add a parser for v2 URLs
  • make sure we handle https://fonts.google.com/xxx URLs correctly, as these can use both the v1 and the v2 format/style syntax
  • unrelated, but we could also parse more than just the font family and weight from URLs - this also applies to v1 (and would fix Some characters look out of place when Latin Extended variant is used in URL #23)
• replace --format with something like --legacy-browsers to download the font with all subsets (or the old API can be used when --format is used)
• add support for display and text parameters
• add support for additional axis like slnt - (https://developers.google.com/fonts/docs/css2#list_of_variable_fonts)
• make sure we handle fallback weights correctly - https://developers.google.com/fonts/docs/css2#legacy_browser_support
• use the new API for downloading

I wrote these notes down mostly for myself to slowly work through, but if anyone else wants to help, that's even better!