Hi esper,
The preview is just a preview of what you have added to the base Webflow CSS stylesheet, which will be included once you download the files.
These CSS stylesheets will be in the CSS folder when you download the zip:
projectname.webflow.css (seen in preview)
normalize.css
webflow.css
You can see limitations here http://forum.webflow.com/t/list-known-limitations-on-webflow/23610?u=samliew