• remove multiple worksheets
• use mime-type output keys
• Remove python-centric names
• Make cell content key uniform
• metadata changes
• Implementation and Coordination
• Full v4 Specification
• Notebook-level format
• Cell-level formats
• Raw cell format
• Markdown cell format
• Heading cell format
• Code cell format
• Output formats
• Execute result
• Display data
• Stream
• Error