It is still an open problem to do unsupervised or mostly-unsupervised part-of-speech tagging that performs as well as (or close to) the usual supervised models.

It is still an open problem to do unsupervised or mostly-unsupervised part-of-speech tagging that performs as well as (or close to) the usual supervised models. For English in the standard domains for which there are annotated corpora this is not necessary, but if you want to apply tagging to a completely different domain or a resource-impoverished language this sort of technique is necessary. This is relevant because it performs just as well (or better) than state-of-the-art methods while being faster and simpler.