here is a better thought: instead of using websites or even their source code you can use a popular wallet to create a paper wallet. wallets such as bitcoin core or electrum.
just download them, verify their signature and then go offline on an airgapped machine. run the wallet and create a new key or better yet create a mnemonic with an HD wallet such as electrum. then write that down on a piece of paper as your paper wallet.
if you like the design that those sites offer you can always find their source code (or even through the HTML in the site that is open) and save the picture which is usually a jpg file and print your key on that.
This is the way to go.
Don't ever use websites to create a paper wallet (neither online nor offline).
The most secure way to generate a paper wallet is to use a live linux distribution on an offline computer.
Either use electrum or any other reputable open source software (signature verified) to generate a private key / mnemonic code or just use openssl from the command line. Both works.
Just don't ever use a website. The risk is way higher and not worth it.