Also one can enter the following in the URL bar:

data:text/html, <html contenteditable>

But it's different from Blank Slate in that it doesn't allow for saving quickly (I make a PDF or take a screen shot if I need what I write for later referencing).

looks like Blank Slate can also do that (notes start as private but you can "publish" them, giving you a public URL). Other cool things about this one are that it is open source (https://github.com/tomasienrbc/notes) and that there are extensions, for instance to turn your note into a Google Doc. It also works with markdown so you can make bulleted lists and such.