Based on this thread: http://mail.openjdk.java.net/pipermail/openjfx-dev/2013-May/007728.html

Basically whenever a number is parsed into the FXML apply the standard CSS units and transform them to pixels before applying them to the item. For example:

<ProgressBar prefWidth="10em" progress="80%" prefHeight="12pt" minWidth="1"/>

All of the attributes would be valid, and parsed according to the CSS transforms done to stylesheets. Prior to this JIRA only the minWidth attribute would be accepted.