Loading Lean 4 + Mathlib into your browser

First visit downloads about 600 MB (the Lean runtime and the full Mathlib environment) and caches it in your browser — later visits start in about 20 seconds. Everything then runs locally: no servers, and your proofs never leave this tab. With the full Mathlib environment loaded, this tab uses about 8–9 GB of memory — on machines with 16 GB or less, close other heavy tabs first.

1. Fetch manifests
2. Install the Lean core library
3. Start the Lean runtime
4. Download the Mathlib environment
5. Load the environment into Lean
6. Check the example proof