The RDFa example for timeRequired contains the following HTML+RDFa fragment:

        Duration: <span property="duration">03:12</span>

This is ambiguous and therefore invalid: A tool wouldn't know whether the 03:12 represents 3 hours 12 minutes or 3 minutes 12 seconds. The corresponding HTML+RDFa should look like this:

Duration: <time property="duration" datetime="PT3H12M" content="PT3H12M">03:12</time>

Similarly, the markup for the following fragment of the same example should be improved:

       Typical length of lesson, <span property="timeRequired">1 hour</span>. 

to

       Typical length of lesson, <time property="timeRequired" datetime="PT1H" content="PT1H">1 hour</time>.