Sheet, a spreadsheet program in 217 bytes of javascript

The code for Sheet fits in one of those newfangled 280-character tweets with room to spare: at 218 bytes, it's the most amazingly compact spreadsheet app committed to screen.

A 218b spreadsheet app in HTML/JS

Inspired by aem1k.com/sheet

Golfed by xem, subzey, p01, rlauck, aemkei, odensc, corruptio
Related AMA answer

See also my new favorite subreddit, r/TinyCode