The GoDoc Playground lets you make changes to your packages documentation and see, in (near) realtime, exactly how this will be rendered on godoc.org.
Automagically fixes unresolved symbols so you don't need to paste in all your code, just a small snippet.
Uses the actual godoc library to parse and render documentation so you can be confident your documentation will be rendered exactly as previewed.
Completely in-browser: uses a WebAssembly compiled parser/renderer so your code never leaves the browser. Even works offline!