Could "code ligatures" be split into a different unicode-range, for font-serving CSS?

[thundernixon]

In Fira Code's font-face CSS from Google Fonts, unicode ranges are split to allow progressive downloads of basic Latin, then Extended Latin, then Greek, and so on.

Fira Code font-face CSS from Google Fonts (Click to expand)

/* cyrillic-ext */
@font-face {
  font-family: 'Fira Code';
  font-style: normal;
  font-weight: 400;
  font-display: swap;
  src: url(https://fonts.gstatic.com/s/firacode/v3/uU9eCBsR6Z2vfE9aq3bL0fxyUs4tcw4W_D1sJV37MOzlojwUKaJO.woff) format('woff');
  unicode-range: U+0460-052F, U+1C80-1C88, U+20B4, U+2DE0-2DFF, U+A640-A69F, U+FE2E-FE2F;
}
/* cyrillic */
@font-face {
  font-family: 'Fira Code';
  font-style: normal;
  font-weight: 400;
  font-display: swap;
  src: url(https://fonts.gstatic.com/s/firacode/v3/uU9eCBsR6Z2vfE9aq3bL0fxyUs4tcw4W_D1sJVT7MOzlojwUKaJO.woff) format('woff');
  unicode-range: U+0400-045F, U+0490-0491, U+04B0-04B1, U+2116;
}
/* greek-ext */
@font-face {
  font-family: 'Fira Code';
  font-style: normal;
  font-weight: 400;
  font-display: swap;
  src: url(https://fonts.gstatic.com/s/firacode/v3/uU9eCBsR6Z2vfE9aq3bL0fxyUs4tcw4W_D1sJVz7MOzlojwUKaJO.woff) format('woff');
  unicode-range: U+1F00-1FFF;
}
/* greek */
@font-face {
  font-family: 'Fira Code';
  font-style: normal;
  font-weight: 400;
  font-display: swap;
  src: url(https://fonts.gstatic.com/s/firacode/v3/uU9eCBsR6Z2vfE9aq3bL0fxyUs4tcw4W_D1sJVP7MOzlojwUKaJO.woff) format('woff');
  unicode-range: U+0370-03FF;
}
/* latin-ext */
@font-face {
  font-family: 'Fira Code';
  font-style: normal;
  font-weight: 400;
  font-display: swap;
  src: url(https://fonts.gstatic.com/s/firacode/v3/uU9eCBsR6Z2vfE9aq3bL0fxyUs4tcw4W_D1sJV77MOzlojwUKaJO.woff) format('woff');
  unicode-range: U+0100-024F, U+0259, U+1E00-1EFF, U+2020, U+20A0-20AB, U+20AD-20CF, U+2113, U+2C60-2C7F, U+A720-A7FF;
}
/* latin */
@font-face {
  font-family: 'Fira Code';
  font-style: normal;
  font-weight: 400;
  font-display: swap;
  src: url(https://fonts.gstatic.com/s/firacode/v3/uU9eCBsR6Z2vfE9aq3bL0fxyUs4tcw4W_D1sJVD7MOzlojwUKQ.woff) format('woff');
  unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+2000-206F, U+2074, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
}

Based on opening a download of the basic latin font in GlyphsApp, it appears that code ligatures are included in the basic latin range. This doesn't seem ideal for the fastest possible downloads – particularly if code ligatures were to be included in a more complex variable font (which is something I am working towards currently).

What might be an appropriate way to subset/split out these ligatures from the first load to improve overall download speed?

(That image may make it look like the code ligatures don't have the correct spacing, but based on a CodePen test, they seem to be working as expected.)

https://codepen.io/thundernixon/pen/OKzVvd

[khaledhosny]

CSS unicode-range is for characters, these are glyphs.

[thundernixon]

Hmm, shoot. So, is the currently no way to progressively load non-unicode characters glyphs such as ligatures?

[khaledhosny]

No, not currently.

[rsheeter]

IIUC this is working as intended. Glyphs that are reachable using the characters in the subset, including by ligature, should be retained.