dev-to-mark-read.user.js

// ==UserScript==
// @name Dev.to Mark as read
// @namespace to.dev.mark-read
// @include https://dev.to/*
// @version 1
// @grant none
// ==/UserScript==

Array.from(document.querySelectorAll('.comment-deep-0')).forEach(el => {
	const toggleReadButton = document.createElement('a');
	toggleReadButton.setAttribute('href', '#');
	toggleReadButton.textContent = 'Toggle read';
	toggleReadButton.addEventListener('click', (e) => {
		let previous = JSON.parse(localStorage.getItem('read-comments') || '[]');
		const id = el.getAttribute('data-comment-id');
		const index = previous.indexOf(id);
		if (index > -1) {
			el.style.opacity = 1;
			previous.splice(index, 1);
		} else {
			el.style.opacity = 0.5;
			previous.push(id);
		}
		localStorage.setItem('read-comments', JSON.stringify(previous));
		e.preventDefault();
	});
	el.querySelector('.actions').appendChild(toggleReadButton);
});

let previous = JSON.parse(localStorage.getItem('read-comments') || '[]');
previous.forEach(id => {
	const comment = document.getElementById('comment-node-' + id);
	if (comment) {
		comment.style.opacity = 0.5;
	}
});