This page compiles MegaDraw.sol with the official Solidity compiler and plays whole rounds against it inside an Ethereum VM, all in your browser. Nothing is sent anywhere. Every round is checked against js/drawmath.js, the same verifier the Provably fair page uses, so the contract and the site are shown to agree.
It downloads the compiler (about 12 MB) from binaries.soliditylang.org and an EVM from jsdelivr, then takes a minute or so.