I am still using Mac OS X 10.9.5 (Mavericks) on one of my machines so as to be able to run older versions of Adobe software etc. I prefer to use Safari as a web browser because it is designed for the Mac, but the final version of Safari for Mavericks (v. 9.1.3) no longer supports certain sites.
These are often ones where an incomplete circular device keeps going round and round without loading whatever it is trying to load. This doesn’t look like a Mac ‘busy’ cursor and it is not browser-specific. An example (trying to load a series of references in response to my query) is shown below (from www.flybase.org) within a red outline I added:
I can work round this using Firefox, but, as an amateur web designer, I am curious to know what the — presumably more recent — web technology is that Safari 9 does not support.