.eleventy.md.js

const markdownIt = require("markdown-it");

module.exports = (eleventyConfig) => {
	let options = {
		html: true,
		breaks: true,
		linkify: true,
	};

	let markdownLib = markdownIt(options).disable("code");
	eleventyConfig.setLibrary("md", markdownLib);

	// Save the original renderer
	const defaultRender =
		markdownLib.renderer.rules.link_open ||
		function (tokens, idx, options, env, self) {
			return self.renderToken(tokens, idx, options);
		};

	// Add attributes for a links
	markdownLib.renderer.rules.link_open = function (
		tokens,
		idx,
		options,
		env,
		self
	) {
		tokens[idx].attrPush(["rel", "noopener"]);
		// Call the original renderer
		return defaultRender(tokens, idx, options, env, self);
	};
};