By default we should always create a new <style> tag.
Appending text to a <style> tag means the contents have to be re-parsed.
Not by the DOM, but for the stylesheet, since the CSS parser can't continue after it ended (it could be left it an open bracket state etc.)
As a result, the browser may have to do various re-paints which are a significant slowdown.
We originally did this "right", but to address the IE stylesheet limit (bug 31676) we changed it to appending text nodes. We should change it back to the right way and only do this hack for IE <= 9.
Modern browsers shouldn't suffer from it.