This adds the possibility for the HTML parser to gather typed items URLs
annotated in HTML tags with itemscope and itemtype attributes (see
microdata specification https://www.w3.org/TR/microdata/ ), notably
Types from the schema.org vocabulary, but also Types/Classes from any
other vocabulary, such as the common ones listed in the RDFa core
context ( https://www.w3.org/2011/rdfa-context/rdfa-1.1.html ).
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