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Let's suppose that, hypothetically, you were playing an online browser-based version of Minesweeper in which there was no capacity to flag a square as a mine. Is there a bookmarklet that would more or less let you do this? (That is, make a small mark somewhere on the screen that you could easily later remove, for the purpose of keeping track of spatial information.)
I haven't been able to come up with any good search terms for finding something like this. The closest thing I can find is a text highlighter.
Unfortunately I need to be able to go back to my browser window to modify content without getting rid of the markers, and it would also be nice to be able to delete arbitrary markers. – Qiaochu Yuan – 2011-07-19T12:30:44.380