go-back-with-backspace/content_script.js

// Listen for shift-backspace or unmodified backspace and navigate if not in
// an editable field.
document.addEventListener('keydown', function(e) {
  if (e.key === 'Backspace' &&
      !e.altKey &&
      !e.ctrlKey &&
      !e.metaKey &&
      !isEditable(e.path)) {
    e.shiftKey ? window.history.forward(): window.history.back();
    e.preventDefault();
  }
});