Loading Third-party JS feedback

[addyosmani]

Philip mentioned that the friendly iframe loader technique is something not captured by the current guide but is valuable for third-parties that can't be pushed out after onload.

https://twitter.com/bluesmoon/status/971423892509208578
http://www.lognormal.com/blog/2012/12/12/the-script-loader-pattern/

Worth us writing a section on for the guide.

[addyosmani]

We also had feedback come in about linking to https://nystudio107.com/blog/a-b-split-testing-with-nginx-craft-cms

[cvazac]

cc @bluesmoon