I think there's some opportunity, if this doesn't already exist, to make some decentralized (WebRTC, with centralized fallback) way of synchronizing localStorage across multiple devices with a pre-shared key. A drop-in script would add functionality to show and scan a QR code or enter a 12 word phrase. It could even be a userscript running without the web app developer's involvement at all. A lot of hoopla for a word game, but could be quite handy in more scenarios.