The read-only basePalette property of the CSSFontPaletteValuesRule interface indicates the base palette associated with the rule.

A string that can be one of the following color values:

light

Matches the first palette in the font file that is marked as applicable to a light background, that is, close to white. If there is no palette in the font or if no palette has the required metadata, the value is equivalent to "0", that is, the first palette in the font.

dark

Matches the first palette in the font file that is marked as applicable to a dark background, that is, close to black. If there is no palette in the font or if no palette has the required metadata, the value is equivalent to "0", that is, the first palette in the font.

a string containing an index (like "0", "1", …)

Matches the palette corresponding to the index. The first palette corresponds to "0".

This example adds rules in an extra stylesheet added to the document, returned as the last stylesheet in the document (document.styleSheets[document.styleSheets.length-1].cssRules). So, rules[2] returns the first CSSFontPaletteValuesRule object, and rules[3] the second one.

HTML

<h2>default base-palette</h2>
<h2 class="two">base-palette at index 2</h2>
<h2 class="five">base-palette at index 5</h2>
<pre id="log"></pre>

CSS

@import url("https://fonts.googleapis.com/css2?family=Nabla&display=swap");

h2 {
  font-family: "Nabla";
}

@font-palette-values --two {
  font-family: "Nabla";
  base-palette: 2;
}

@font-palette-values --five {
  font-family: "Nabla";
  base-palette: 5;
}

.two {
  font-palette: --two;
}

.five {
  font-palette: --five;
}

JavaScript

const log = document.getElementById("log");

const rules = document.styleSheets[document.styleSheets.length - 1].cssRules;
const twoRule = rules[2]; // A CSSFontPaletteValuesRule interface
const fiveRule = rules[3]; // A CSSFontPaletteValuesRule interface

log.textContent = `The ${twoRule.name} @font-palette-values base palette is: ${twoRule.basePalette}\n`;
log.textContent += `The ${fiveRule.name} @font-palette-values base palette is: ${fiveRule.basePalette}`;

Result

• @font-palette-values at-rule
• base-palette descriptor