I originally responded to a claim that the pattern of ERVs indicates common design as much as it does common descent.
I disagreed because ERVs are not a "design feature". That is what sets them apart from say shared functional genes between species which can be said to be a design feature.
You question how I know ERVs are not a "design feature". I guess I don't absolutely know they are not, but there is no more reason to think they are than to think the pattern of craters on the moon is a design feature, or the shape of the himalayas is a design feature.
The pattern of ERVs fit a nested heirarchy. Common descent expects this, common design does not.
Let's say you're programming in an object oriented language, and you have a parent class which contains some, err, poorly developed code. If you make a child class which extends the parent class, you will bring the poor code along.
Parent / child = descent. But it's still design in that case.
Just stirring the pot.
Cheers!