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Invisible data or metadata for that matter is undesirable.
See Principles of visibility and human friendliness.
Also known as, "dark data", "meta tags", "side files".
invisible metadata failures
- meta keywords
- popular in the 1990s, eventually spammed, and rotten, ignored by Google, Yahoo, and other search engines.
- meta ICBM
- people move, don't bother to update their meta ICBM
- people get it wrong (because it's not obviously visible)
- swapping lat and long
- "correcting" negative values to positive (or vice versa)
- note clusters of sites in the middle of oceans or other open spaces that correlate with inverse (or negated) coordinates of actual cities
- lang="en"
- lots of templates, CMS's shipped with this default
- people used them worldwide
- now lang="en" is effectively meaningless/untrustworthy since tons of non-en sites all have it (from abovementioned templates etc.)