A Google Reader Help thread has a request from one Google Reader user that he would like if Google Reader would support HTML5 elements. He specifically asked that the audio and video elements would be supported.
A Google Reader support representative, Ella, said:
Thanks for bringing this up! This is a great idea -- it's on our radar as we continue to develop features for Reader :)
There is no doubt that HTML5 is on Google's radar, the main question is how soon will Google Reader and other properties and products support HTML5?
I think sooner than later.
Forum discussion at Google Reader Help.