Type the following text into the address bar of your browser.
data:text/html;charset=utf-8, <title>TextEditor</title><body contenteditable style="font-size:2rem;font-family:monaco;line-height:1.4;max-width:60rem;margin:0 auto;padding:4rem;" spellcheck="false"><h1>Text Editor</h1><p>Start Here.
Voila, a text editor inside your browser.
Thanks to Jose Jesus Perez Aguiaga and the commenters. There may even be better suggestions in the comments later on.