Behind every door is a festive treat all about CSS-grid!
This page uses display:grid on an unordered list to set a series of list items as equally proportioned advent calendar "doors". Just like a real advent calendar, the doors are shuffled out of order by calling each into a specific position on the grid.
If the worst comes to the worst and you can't see the grid the page will just fall back to a bullet list, with all the treats in sequence—may you get a new browser from Father Christmas this year.
Inside each door is one of 25 daily treats which appear over the main page—nested list items are hidden / displayed when you click on their parent door on the calendar.