Comment by marginalia_nu

When designing a standard like this you need to apply systems thinking.

The poor data quality is a problem for anyone who wants to use the tags, which has seen everyone almost universally reaching for other solutions. Search engines have preferred anchor tags, bookmarking solutions have applied user tagging.

With keyword tags it's been a vicious circle of poor data quality and neglect since day one. Even in documents from the early 1990s when people were really trying, the data quality is inconsistent at best.