Check out some problems from the MODS POTD here. Use Shift+Click to highlight a problem you've done, and Ctrl+Click to star a problem with a different color. To mark a problem as TODO, use Alt+Click. This was originally meant to be a hidden page since opening a problem normally reveals my thought process, solution, and the idea I found interesting behind it, but I ended up making it public anyway.
Once more, warning: Haven't implemented spoilers, so if you click into a problem to view it, you might end up seeing the solution. To know which problems I may have written stuff on, read below paragraph.
In order to import what problems you've solved over from another browser or something like that, use the Export button on the page where you have everything marked, and Import the json file you get from that into the new page. Here's my marked list: TODO.json.