Here’s a small cool trick to make the whole document editable.
document.designMode = 'on';
This can also be used with iframes, i.e.
let iframeNode = document.getElementById('myeditor'); // or any other ways to get reference to an iframe iframeNode.contentDocument.designMode = "on";
Try this out with the current page by clicking here!
Did you know that I made an in-browser image converter, where you can convert your images without uploading them anywhere? Try it out and let me know what you think. It's free.