I think not. I don't think it would make any sense in the context of JavaFX. It makes modifications to html elements which don't exist in JavaFX.
I guess it would work in the context of a css file referenced by an html file loaded by webview, but that is not your intended use is it?
If you could specify a content attribute, what would you have it do?
Can you provide an example of a JavaFX program+css file in which you make use of the content attribute?
Here is a reference to the content attribute for those interested: http://www.htmldog.com/reference/cssproperties/content/