Feature Strings

OpenType features that need to be applied during shaping by Harfbuzz can be specified as a list of comma-separated strings. Details of the format are given below.

References

• Registered features in OpenType spec

From Harfbuzz documentation

Here is the Harfbuzz documentation for specifying feature strings, taken from the help output of the hb-shape tool.

    Features can be enabled or disabled, either globally or limited to
    specific character ranges.  The format for specifying feature settings
    follows.  All valid CSS font-feature-settings values other than 'normal'
    and 'inherited' are also accepted, though, not documented below.

    The range indices refer to the positions between Unicode characters,
    unless the --utf8-clusters is provided, in which case range indices
    refer to UTF-8 byte indices. The position before the first character
    is always 0.

    The format is Python-esque.  Here is how it all works:

      Syntax:       Value:    Start:    End:

    Setting value:
      "kern"        1         0         ∞         # Turn feature on
      "+kern"       1         0         ∞         # Turn feature on
      "-kern"       0         0         ∞         # Turn feature off
      "kern=0"      0         0         ∞         # Turn feature off
      "kern=1"      1         0         ∞         # Turn feature on
      "aalt=2"      2         0         ∞         # Choose 2nd alternate

    Setting index:
      "kern[]"      1         0         ∞         # Turn feature on
      "kern[:]"     1         0         ∞         # Turn feature on
      "kern[5:]"    1         5         ∞         # Turn feature on, partial
      "kern[:5]"    1         0         5         # Turn feature on, partial
      "kern[3:5]"   1         3         5         # Turn feature on, range
      "kern[3]"     1         3         3+1       # Turn feature on, single char

    Mixing it all:

      "aalt[3:5]=2" 2         3         5         # Turn 2nd alternate on for range

From CSS Font

As mentioned above “All valid CSS font-feature-settings values other than 'normal' and 'inherited' are also accepted”. Shown below is a sample CSS document containing some examples.

/* use small-cap alternate glyphs */
.smallcaps { font-feature-settings: "smcp" on; }

/* convert both upper and lowercase to small caps (affects punctuation also) */
.allsmallcaps { font-feature-settings: "c2sc", "smcp"; }

/* enable historical forms */
.hist { font-feature-settings: "hist"; }

/* disable common ligatures, usually on by default */
.noligs { font-feature-settings: "liga" 0; }

/* enable tabular (monospaced) figures */
td.tabular { font-feature-settings: "tnum"; }

/* enable automatic fractions */
.fractions { font-feature-settings: "frac"; }

/* use the second available swash character */
.swash { font-feature-settings: "swsh" 2; }

/* enable stylistic set 7 */
.fancystyle {
  font-family: Gabriola; /* available on Windows 7, and on Mac OS */
  font-feature-settings: "ss07";
}