Our in-browser text editor does not work with the 14.0.3 version of Safari that comes with macOS Big Sur 11.2.3.
The solution is to either...
1. Upgrade to the latest version of macOS Big Sur (11.4), which includes Safari 14.1.1. Our in-browser text editor works in this version of Safari. (We've also tested in macOS Big Sur 11.3, and our in-browser text editor works with the version of Safari that ships with this version of macOS Big Sur.)
2. Switch to Google Chrome.