http://build.fluidproject.org/infusion/demos/uiOptions/demo.html

If you take a look at the demo and try inspecting drop-down controls within one of the UI Options panels you will notice that the there is a label for a drop-down (for example: <label for="theme-selection">Colour & Contrast</label>) that has points to a select attribute with missing id (e.g. theme-selection).