Opera Sticky Notes
15th July 2004 · Last updated: 5th October 2016
Here's the standalone demo - the sticky note is not in the HTML, but entirely generated from CSS.
To try it out, download this stylesheet and save it in the directory given in your Opera Preferences for Page Style, where it says "My style sheet". (You might need to create the "styles" folder first.) Make sure you then click "Configure Modes" and tick "My style sheet" in the Author Mode list. Click "Apply" also to make it work. (Note: if you change the user style, untick and tick it again and re-apply it to show the changes.)