1. Create an application that has a button called 'Increment' and shows a number. Every time the button is clicked the number increases by 1. Implement it in a single HTML file without a 'script' tag.
2. Add another button called 'Decrement'. When the user clicks that button the counter is decremented. Still have this in a single HTML file without a 'script' tag.
3. Change the application now moving everything to a controller. Both the initialization and the code of increment and decrement. Disallow negative numbers: If the user clicks on 'decrement' while the counter is at 0, display a message in a 'div' element that this operation is not allowed.