That project is based on the original Ian Coleman BIP39 tool. There haven't been any commits in the last 2-3 years, but it should still be working. You can use it to check if your Electrum seed is correct and if it's generating the correct addresses. Just make sure to download and run it offline like you would do with Ian Coleman's tool as well.
Rather than use a tool which is 3 years out of date and forked from a version of Ian Coleman which is also 3 years out of date, better just to do it yourself with the latest version of the software, so you know you are not risking being subjected to any bugs or vulnerabilities. It takes 2 minutes to do.
Go to Ian Coleman's site and download the zip of the latest version. Extract and navigate to \src\js, and then open the file jsbip39.js with a text editor.
Navigate to line 116:
return h == nh;
Change this line to the following:
return h == true;
Navigate to line 144:
passphrase = "mnemonic" + passphrase
Change this line to the following:
passphrase = "electrum" + passphrase
The first change tells it to ignore the checksum since you are using an Electrum seed phrase and not a BIP39 seed phrase. The second change fixes the salt that Electrum uses when turning seed phrases in to private keys. That's all you need to do. Then just go back in to the \src\ folder and run index.html, and you'll have a working Electrum seed checker.