Type a mark and it saves straight away. Leave a cell empty for work that has not been handed in. The percentage column updates as you go and ignores empty cells, so an early mark is not dragged down by work that does not exist yet.
A file format and a grid. The export is plain JSON that you can open in any text editor and read, which means you are not locked into this page and nothing can hold your marks hostage. Importing replaces everything currently in the browser, so export first if you have unsaved work on this device. There are no accounts, no sync, no backup and no undo. The honest reason this exists is that a teacher should be able to keep a term's worth of marks without handing them to a company, and the honest cost of that is you have to press export.