CSS files are send as text/html causing problems if the HTML page is rendered in standards mode. The HDFS interface for example still works because it is rendered in quirks mode, the HBase interface doesn't work because it is rendered in standards mode. See
HBASE-2110 for more details.
I've had a quick look at HttpServer but I'm too unfamiliar with it to see the problem. I think this started happening with
HADOOP-6441 which would lead me to believe that the filter is called for every request and not only *.jsp and *.html. I'd consider this a bug but I don't know enough about this to provide a fix.