Inspired by: https://terkel.jp/archives/2019/11/font-feature-settings-custom-properties/

tailwind-font-feature-settings-plugin.js

const plugin = require("tailwindcss/plugin");

const fontFeature = plugin(function () {
	const tags = {
		locl: 1,
		palt: 0,
	};

	const cssFontFeatureSettingsValue = Object.keys(tags)
		.map((tag) => `"${tag}" var(--tw-${tag})`)
		.join(", ");

	const defaults = {};
	for (const [key, value] of Object.entries(tags)) {
		defaults[`--tw-${key}`] = value.toString();
	}

	const utilities = {};
	for (const tag of Object.keys(tags)) {
		utilities[`.${tag}`] = {
			"@defaults font-feature-settings": {},
			[`--tw-${tag}`]: "1",
			"font-feature-settings": cssFontFeatureSettingsValue,
		};
		utilities[`.not-${tag}`] = {
			"@defaults font-feature-settings": {},
			[`--tw-${tag}`]: "0",
			"font-feature-settings": cssFontFeatureSettingsValue,
		};
	}

	return function ({ addDefaults, addUtilities }) {
		addDefaults("font-feature-settings", defaults);
		addUtilities(utilities);
	};
});