The tool.css import in the header fragment of an HTML doc created with the resources tool does not account for the neo prefix.
Because of the issue identified in
SAK-27873, portal.inline.experimental=false needs to be set to reproduce this in a trunk instance.
1. Navigate to the resources tool of a site.
2. Click Actions -> Create HTML Page
3. Add some content to the editor then click Continue.
4. Give the page a name then click Finish.
5. Link the page with the web content tool or download the html file if content.html.forcedownload is true.
If you download the file and open it up in a text editor the neo prefix will be missing from the path to the tool.css file.