Keep in mind that it is unlikely that any elements will be changing in size without a window resize, text size change or zoom change so firing onResize on the elements for those events would minimize any performance hit. Now there is still the case of AJAXed in content, modifying the DOM on the fly with JavaScript and other things that could change the size of an element. In those cases you could always have plugins manually trigger the element-reszie-width.wb and element-resize-height.wb events if it turns out to be too much of a performance hit to go with the polling approach.