Currently, they'd all be modifying the same cookie, but it would be cool if the user could save and retrieve multiple docs

the bare textarea is not great. Prettyify it like with stackedit.js

Create shortlinks for these awfully long links, probably through a third party service.

Split up the HTML, JS and CSS like any modern project should

Insert emoji in the place of colon emoji like :joy:. Optional escaping when in code blocks.

https://stackoverflow.com/questions/6140632/how-to-handle-tab-in-textarea but without jquery

Could mitigate XSS vulnerabilities, modeling off of this issue in itty-bitty

Chrome 69 will start denying replaceState calls to keep the browser from hanging, but this means that lots of typing at once may not be captured correctly. Need to think of a workaround -- one thought is to rate-limit the calls ourselves, another could be to periodically make the replaceState call instead of doing it on every input to the box.

Perhaps the content could be built into the URL, like itty.bitty.site

Add checkbox to XSS warning, saying "Do not show again".