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Do you have JavaScript enabled? It is needed to make the hidden tags work. I don't have access to an iOS device currently but I'll get to checking to see if it's a problem with iOS itself or not. EDIT: just noticed you said highlighting. Have you clicked/tapped on the blacked out text? How the tag works is that it remains hidden until it is clicked/tapped and it then reveals the text. Another click then re-hides it.