It was initially a Chrome extension [1] but there were limitations with local file access and UX from jumping around to multiple pages. Very early on I developed this as a web app but it was also difficult to inject styles into iframe, especially for ones that we didn't own directly.
Electron ships Chromium by default so it was easy to leverage into an editable browser plus allowing us to do an infinite canvas style editor. If you know of ways around these limitations please let me know. I'm always trying to figure out a way to turn this into a web app instead.
[1] https://chromewebstore.google.com/detail/onlook/icbcddooibfg...