“SingleFile is a Web Extension (and a CLI tool) compatible with Chrome, Firefox (Desktop and Mobile), Microsoft Edge, Safari, Vivaldi, Brave, Waterfox, Yandex browser, and Opera.”
Browsers have had “Save Page As” for decades and it still dumps a folder of broken assets next to a half-working HTML file. One developer fixed it with an extension that inlines everything into one file you can open anywhere. Pages vanish, paywalls go up, and links rot. Save the copy yourself, because nobody else is keeping it for you.