Any id that starts with three characters that are all in the range of a-f will cause a ParseException in the CSS parser. For example, any of the following will cause an exception: aaa, add, bad, faa, fab.

The exception will be thrown regardless of what characters follow the first three: fabulous and addIsBad will cause the exception. An id with 1 or 2 characters are unaffected: a and aa are fine.

This does not affect style classes.

Test code:

// Main.fx
Stage {
    title: "CSS Bug Test"
    scene: Scene {
        width: 100
        height: 100
        stylesheets: ["{__DIR__}addtest.css"]
        content: Circle {
            id: "addIsBad"
            styleClass: "aaaIsFine"
            centerX: 50
            centerY: 50
            radius: 40
        }
    }
}

// addtest.css
Circle {
    fill: cornflowerblue;
    stroke: blue;
}

Circle.aaaIsFine {
    fill: yellowgreen;
    stroke: green;
}

Circle#addIsBad {
    fill: pink;
    stroke: red;
}