Things to do:

(Not necessarily in priority order)

Investigate check reporting "may not have a unique solution" when no solution is possible
Menu option to share with pencilmarks
Investigate honouring back button to close modal/menu (without propagating state when sharing current URL)
Tests for UI
Allow pasting in a digit string in 'enter' mode
Allow copy to digit string
Review/playback mode with slider and step buttons
Allow contributed themes
Add native sharing implementation (https://css-tricks.com/on-the-web-share-api/)

Things done

Switch from deprecated document.fullscreen to document.fullscreenElement
Track progress in localStorage to allow resume after reload or back/fwd
Protect against accidental reload or back button with "are you sure you want to leave this page?"
Tests for model
Render cell layout and text using SVG+CSS
Mouse input for selecting cells
Keyboard input for entering/clearing digits
Keyboard input for selecting cells
Inner & outer pencil marks
Enter key to trigger a check & highlight errors
Undo/redo support
Set initial digits from querystring
Input mode for setting initial digits
Timer
Highlight matching digits & pencil marks
On-screen buttons for mobile/touch input
Adapt screen layout for portrait vs landscape
Implement restart
Colour completed digits on virtual keyboard
Cell colouring
Pause timer
Menu option to clear all pencilmarks
Implement touch event handling
Allow multiple-cell selection on mobile/touch
Auto clean pencilmarks
Double-tap digit to switch to digit mode
Button to go fullscreen
Dark mode theme
Fun visual effect when puzzle is solved
Block starting if grid has errors
Validation of digit string in URL (81 digits + no conflicts)
Settings dialog with selections persisted to LocalStorage
Help text
Function to check for unique solutions (new mode)
Sharing options via social media
Allow user to share difficulty level and/or solve time
Add modal to allow selecting from recently shared puzzles
Option to share via QRcode