Create Nested Frameset in HTML and CSS. By replacing the element in the document structure, this element defines the document layout in terms of areas of the screen containing sub-document URLs (the frame content). The simplest way to get a combination of rows and columns is to first set up the "outermost" set of rows or columns, and then replace the FRAME element in the row (or column) which should get the "innermost" set with a nested FRAMESET element. BoxAdcontent.document.write("