Yes. An md reader that renders files entirely in your browser works offline once the page has loaded: the Markdown is parsed on your device, not on a server, so a saved tab keeps reading your file with the network switched off. Drop in a .md file and it renders instantly — nothing is uploaded, so it stays private even when you are back online.
Open the viewer — drag in your .md file →Most "online md readers" send your file to a server, render it there, and return HTML — no server, no preview. This one parses Markdown locally in the tab, which means three things: it keeps working on a plane or in a dead-zone, your file never leaves your device (no upload, no retention, no account), and a 10 MB README renders at the same speed as a 10 KB one because there is no network round-trip. Reload behaviour: after the first visit the app shell is cached by the browser, so opening the page on a flight still works — drag your file in and read.
.md / README.md file onto the drop zone (or click to browse).