gwt / gwt-query - 在面板内部创建透明的一些内部区域

I've been looking forward something that enables me to transparent some inner areas inside a panel but not the rest. Say, if I have a panel which dimensions are 500x300, I would like to transparent only the area which left and top (relative to its parent) are 30 and 40 respectively and which width and height are 100 and 200 respectively. I've been looking forward something that enable