The link to the CSS reference guide from https://openjfx.io/javadoc/11/javafx.controls/javafx/scene/control/package-summary.html gives a 404, pointing to https://openjfx.io/javadoc/11/javafx.controls/javafx/scene/doc-files/cssref.html.
The file javafx.controls/javafx/scene/control/package-summary.html links by using href="../doc-files/cssref.html".

This is a result of the javadoc restructuring to organize docs by module (JDK-8195795) for the JDK 11 javadoc tool. The javafx.controls package docs refers to the cssref.html guide as "../doc-files/cssref.html". Since they are now in different modules, it is necessary to include the module name in the path, like this:

href="../../../javafx.graphics/scene/doc-files/cssref.html"