Sheet JS is quite amazing, but another, simple approach to consider is based around the fact when you copy a range from Excel, it's plain-text clipboard format

Sheet JS is quite amazing, but another, simple approach to consider is based around the fact when you copy a range from Excel, it's plain-text clipboard format is tab-separated values. Get the users to copy their data and paste it into a text box with an on-paste handler that parses the data into a preview table with validation so they can see if they've got the columns right before hitting submit. Simple, 100-ish lines of JS, broad browser support.