One of my dreams if I were to create my own web-based spreadsheet engine would be to make worksheets much more general purpose and not stuck in tab UI like Excel. You would be able to embed them separately in the page so multiple worksheets could be viewed at the same time. Example: you might have multiple small worksheets with financial summaries where the columns are slightly different. You could also do things like include text above/below worksheets, to present the data in a document-like format.