I think in this case, it sounds like you’re looking to build a generic NFT Marketplace contract, there are many templates for this already available. Most of these would be built as an extension of the IERC721.sol interface.
The only thing your web app will be doing is ensuring the user has some wallet connected to it (you can use something like rainbowkit for that).
So to return to your initial question; the ‘Celo Standard’ is the same as the Ethereum standard for NFTs (ERC721, ERC1155 OpenZeppelin standards as 0xGoldo pointed out).
You can extend these with other functionality as you require. But these should be enough to create basic ones for your web app.
If you’re then hoping to be able to mint NFTs on your own web app that can also be sold to a different NFT Marketplace (like OpenSea for example, you would need to ensure the metadata for your NFT matches that of the desired platform’s specification [see for example: Metadata Standards] to make the integration possible)