Gatsby's gatsby-plugin-offline allows websites to function offline by preloading pages and resources, even when JavaScript is disabled. The plugin, which can be easily integrated into existing Gatsby sites, leverages service workers to provide offline access by caching pages and serving a lightweight "offline shell." The recent release of version 3 introduced enhancements such as a major update to Workbox and a new feature that ensures full pre-rendered HTML is served when JavaScript is disabled, improving accessibility. This functionality is achieved through a communication mechanism between service workers and page-level JavaScript, which uses message APIs and specially formatted URLs to detect JavaScript status and adjust the site's offline behavior accordingly. The plugin can be tested by adding it to an existing site or creating a new one using the Gatsby CLI, while maintaining website performance and accessibility.