To reproduce, run HelloHTMLEditor with a security manager enabled:

java -Djava.security.manager hello.HelloHTMLEditor

The HTMLEditor control will fail to load the resources from the modena.css file and will produce the following warnings:

Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Cut.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Copy.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Paste.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Left.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Center.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Right.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Justify.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Outdent.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Indent.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Bullets.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Numbered.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Text-Color.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Background-Color.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Bold.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Italic.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Underline.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Strikethrough.png
Mar 20, 2017 2:40:13 PM com.sun.javafx.css.StyleManager getCachedImage
WARNING: Error loading image: jrt:/javafx.controls/com/sun/javafx/scene/control/skin/modena/HTMLEditor-Break.png

A default set of resources is used.

I tested it on JDK 8 and it fails there, too (the URL to the resources is necessarily different, since it uses a jar: URL rather than the new-in-9 jrt: URL).