Ok this is a weird one.
I have a nest registered board and I'm connected to the IDE via the local URL. I have an SD card installed and I have a HTML, a CSS, and nut in the directory sd/lib/demos/pool/.
If I edit the CSS file, save it, close it and then reopen it, my changes are gone. Same with the HTML file. If I edit the nut, changes are saved as expected.
Wait it get's stranger.
If I go back to nest and open the IDE via the cloud link, my changes are there. Going back to local they are gone again.