Beyond W3C XML Schema

Datatypes everywhere Data typing isn't a concept unique to W3C XML Schema. Formally or informally, lots of specs have defined data types to meet their need. RELAX NG, etc. reuse W3C XML Schema types, or a limited subset, but others (like SOAP) meander.
The HTML style attribute The HTML style attribute can contain lots or little Cascading Style Sheets information. style effectively isolates presentation information, but uses lexical notation rather than markup to describe complex information. As a result, some folks consider it an abomination.
Lexical notation is human Yes, I live and breathe in angle brackets, but most people don't. People are used to a wide variety of notation, from simple dates to stock-tickers to mathematics. Accomodating people seems wiser than trying to fit their heads into our vises.
How far can markup go? If you need markup which essentially duplicates the information inherent in content notation (think every word in every sentence), the redundancy starts to get painfully obvious. Automating processing so that developers can use content as markup without have to explicitly mark it up seems a worthy time-saver.