I was working with Infragistics WebHtmlEditor control, and on client side it was being moved around by appendChild, removeChild DOM methods. All worked fine until I tried it over SSL connection. Immediatly removeChild method cause Secure/Unsecure waring.  Digging a bit I found that it was an IE bug. According to Microsoft this problem occurs if the Web page script calls the removeChild method to delete a DIV element that references a background image, and they suggest either to set outerHTML of the DIV to an emtpy string (which I think is dumb if you still need to use element you removed), but their second solution to move background-image declaration into an external CSS class actually works. In reality this problem occurs not only with DIVs but with any HTML element with background image in its inline style. In my case WebHtmlEditor rendered itself as an HTML table and its cells had background-image specified. The control itself didn’t have corresponding images set, but its UseDefaultStyles property was set to BackrgoundImages – which resulted in rendered styles with background images. To apply modified Microsoft’s solution I set this property to None, then segregated background image into a separate class:

.HTML_EDITOR {
   background-image: URL(ig_common/images/htmleditor/backimagerow.gif)
}

and used that class in Toolbar and TabStrip CSSClass properties. Worked like a charm.