Tags → phpStorm 
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phpStorm: URI is not registered (Settings | Languages & Frameworks | Schemas and DTDs)
So I was looking through ancient HTML files last night, as one does, and noted this peculiar error in phpStorm around the DTD of one of the files: <!DOCTYPE HTML PUBLIC “-//W3C//DTD HTML 4.01 Transitional//EN” “http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd”> phpStorm did not like this DOCTYPE. It reported to me: URI is not registered (Settings | Languages & Frameworks…