The code example illustrates annotation of a short text about a chemical substance. This code employs microdata (property/value pairs) according to the vocabulary defined at www.axeleratio.com/voc/chemid. The W3C validator complains about the item attributes, declaring that their use is not allowed on the selected elements at this point. In contrast, the (X)HTML5 Validator at html5.validator.nu declares that this code is valid HTML5, after submitting a page containing this code snippet. The NU Markup Validation Service at qa-dev.w3.org:8888/html5 came up with the same result. The latter validator—according to the statement on its site—checks the markup validity like the current W3C validation service, but instead uses the backend of the Validator.nu engine, which provides non-DTD-based validation support for a number of markup languages. Currently, this latter service is a potentially unstable pilot version, provided for demonstration and testing purposes only.
Suggested sites for microdata digging:
Tutorials Point: www.tutorialspoint.com/html5/html5_microdata.htm