In short, it would be nice to have an instruction like <lx:require-resource src="url" type="mime"/> with the same arguments as \lxRequireResource. In fact, it would simply reduce to a call to copyResource.

The benefit is that different stylesheets would be able to programmatically include additional CSS, Javascript, images, etc. which would be very helpful when generating different outputs from the same XML file. Right now, one can pass additional files as arguments to latexmlpost, but the functionality is limited (e.g. you cannot customise the location of the files, one cannot pass images or other resources, and so on).