html

<pre id="log"></pre>

css

/* At-rule for "nice-style" in Font One */
@font-feature-values Font One {
  @styleset {
    nice-style: 12;
  }
}

/* At-rule for "nice-style" in Font Two */
@font-feature-values Font Two {
  @styleset {
    nice-style: 4;
  }
}

/* Apply the at-rules with a single declaration */
.nice-look {
  font-variant-alternates: styleset(nice-style);
}

js

const log = document.getElementById("log");
const rules = document.styleSheets[document.styleSheets.length - 1].cssRules;

const fontOne = rules[0]; // A CSSFontFeatureValuesRule
log.textContent = `The 1st '@font-feature-values' family: "${fontOne.fontFamily}".\n`;

const fontTwo = rules[1]; // Another CSSFontFeatureValuesRule
log.textContent += `The 2nd '@font-feature-values' family: "${fontTwo.fontFamily}".`;