Audit: SPOF detection

Each blocking origin in the critical rendering path is an invitation for a SPOF (single point of failure).

##### Resources:
* [Frontend SPOF | High Performance Web Sites](https://www.stevesouders.com/blog/2010/06/01/frontend-spof/)
* [Avoiding a Single Point of Failure • A Faster Web](https://www.afasterweb.com/2015/08/15/avoiding-a-single-point-of-failure/)
* [Performance Matters: Testing for Frontend SPOF](http://blog.patrickmeenan.com/2011/10/testing-for-frontend-spof.html)

##### Tools:
* [SPOFCheck](https://senthilp.github.io/spofcheck/) and writeup: [Early Detection of Frontend Single Points of Failure | eBay Tech Blog](http://www.ebaytechblog.com/2013/01/22/early-detection-of-frontend-single-points-of-failure/)
* [WPOTools/spof: Spof your site.](https://github.com/WPOTools/spof)
* [SPOF-O-Matic - Chrome Web Store](https://chrome.google.com/webstore/detail/spof-o-matic/plikhggfbplemddobondkeogomgoodeg?hl=en-US)