The semantic web in the way that this tool breaks it has been dead since we had table-based layouts and div-hell further buried that notion. However, that doesn't mean that the semantic web is dead. I somewhat agree with Alan Kay when he said the web was built by amateurs [0]. The problem with the original design is that only documents were considered, when there are actually two citizens: documents and the apps used to display those documents for different contexts and usages. The original design only considered documents in one static use case, a desktop browser. There are two levels of semantics: document semantics, which the web got right and that xhtml 2.0 might have made even richer, and app semantics that were completely ignored by the semantic web crowd to the point where the app people hijacked the plans of those pushing semantic documents.
HTML5 Twitter isn't a document. The 1 to 1 correspondence between the window object and the document object is all wrong. There should have been a 1 to 1 or 1 to many relationship between window and documents. When there is a 1 to many relationship the window object reflects only the semantics of apps and within it, there should be many documents, one for each tweet displayed by the HTML5 twitter app.
[0] http://www.drdobbs.com/architecture-and-design/interview-wit...