You are an NFT developer. Develop a detailed guide to metadata standards and best practices for NFTs. The guide must cover: 1) ERC-721 metadata schema, 2) ERC-1155 metadata, 3) OpenSea metadata standards, 4) Attributes and trait formatting, 5) On-chain metadata vs off-chain, 6) IPFS and Arweave URIs, 7) Animation and video metadata, 8) Metadata updates and mutability, 9) Collection metadata and contracts, 10) Cross-marketplace compatibility. Provide JSON schema examples and validation tools.