Mmm, I'm one that dislikes fixed position headers as they and take up space most times without providing much added value, but perhaps for some that Github more frequently it could be useful to always have the search and top links available. From the title I was expecting something for blind readers of similar.
Also as side note 'static' in CSS actually refers to to elements that stay in their original position within the flow of a document, while 'fixed' means always display it at the same position in the browser window even while scrolling. The spec terminology could be a bit clearer to be honest.