Had same problem. Try (,or something like) this (named "Bocs" instead of "Box") to avoid name clashes in future):
public class Bocs extends WebMarkupContainer { public static final byte DEFAULT_ROUNDING_RADIUS = 8; public static final String DEAFAULT_FILLCOLOR = "#EEFFD0"; public static final int DEAFAULT_BORDERWIDTH = 1; public static final String DEFAULT_BORDERCOLOR = "#AAAAAA"; public Bocs(String id) { this(id, DEFAULT_ROUNDING_RADIUS, DEAFAULT_FILLCOLOR, DEAFAULT_BORDERWIDTH, DEFAULT_BORDERCOLOR); } public Bocs(String id, int roundingRadius, String fillColor, int borderWidth, String borderColor) { super(id); this.setOutputMarkupId(true); this.setOutputMarkupPlaceholderTag(true); // CAUTION: do not substitute Graphics graphics = new Border(borderColor, borderWidth); graphics.setChainedGraphics(new Fill(fillColor)); LiquidCanvasBehavior behavior = new LiquidCanvasBehavior(graphics, new RoundedRect( (byte) roundingRadius)); this.add(behavior); } private static final long serialVersionUID = 1L; } Use latest Wicket (1.4-rc5), Wicketstuff Artwork (1.4-20090706.220716-150). Stefan