While you can't save to localStorage as my sibling commenters have shown, greyfa...

While you can't save to localStorage as my sibling commenters have shown, greyface- down below in the thread posted a version that saves to the hash fragment of the URI. Saving to the (Data) URI has a benefit over localStorage of allowing you to save by bookmarking, which also enables you to save many notes, not just one.

I code-golfed greyface-'s code and made the text cursor autofocus on page load:

  data:text/html,<body contenteditable autofocus oninput="history.replaceState(0,0,'%23'+btoa(this.outerHTML))" onload="location.hash&& document.write(atob(location.hash.slice(1)))">#