I've seen a lot of reports recently (most recently in the ControlsFX project here: https://bitbucket.org/controlsfx/controlsfx/issue/370/using-controlsfx-causes-css-errors-and ) about JavaFX custom controls showing runtime warnings and visual glitches. This seems related to the use of Control.getUserAgenetStylesheet(), which is not something we use in JavaFX itself, but which is used by third party controls to bring in custom stylesheets. After discussing with David it seems best to improve the CSS engine to improve the handling of this code, and to isolate the stylesheets brought in by a Control such that they only impact themselves and their children. This prevents a bad stylesheet breaking other aspects of the UI. This seems both reasonable and sensible.