It is possible to define custom elements once and use it through the entire site.
It is possible to use custom elements inside another custom element.
It is possible to use styles inside custom elements.

To define: in the configuration file, in /website/config/sys/elems, define custom element called TopNav_el_nrm

<TopNav_el_nrm eltype="elem_group" x="0" y="0" x2="R" y2="B" >
	<el eltype="rect" x="4" y="0" x2="R-4" y2="B" c="0xdddddd" a="100" />
	<el style="TopNav_txt_nrm" x="20" />
</TopNav_el_nrm>

To use:

<el elem="TopNav_el_nrm" />

As with styles, it is possible to overwrite any attribute when using the custom element.

For example, see source code at
http://ximl.com/sites/ximl_tabs1/conf.xml