And separating content from presentation is just an example of the more general pattern of separating data from application, which is one of the rationales for databases (and indeed most multi tier systems). What Goldfarb et al realized in the 70s at IBM was not really a new pattern, but that documents also fitted into the emerging two(+) tier architectures (and, indeed, that document longevity forced, to some extent, a virtual single-input/multiple-application architecture-- even if only one application was active in any given period there would still need to be a succession of applications as technology moved onwards, and marking up documents to make this succession as easy as possible would be an investment and a risk reduction.)
Cheers
Rick Jelliffe