Per T112593 and 5b5621c3199d we now measure the rejections from the `isCompliant()` check.
It turns out about 5-10% of Navigation Timing events are ignored client-side (~ 20 out of 300 every minute).
Here's a 1 minute sample of User-Agent values from the Kafka stream for statsv with metric_name `eventlogging.client_errors.NavigationTiming.nonCompliant`.
```
"user_agent":"Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)"
"user_agent":"Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)"
"user_agent":"Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; WOW64; Trident/6.0)"
"user_agent":"Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; WOW64; Trident/6.0)"
"user_agent":"Mozilla/5.0 (compatible; MSIE 10.0; Windows Phone 8.0; Trident/6.0; IEMobile/10.0; ARM; Touch; NOKIA; Lumia 520)"
"user_agent":"Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)"
"user_agent":"Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)"
"user_agent":"Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)"
"user_agent":"Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)"
"user_agent":"Mozilla/5.0 (compatible; MSIE 9.0; Windows Phone OS 7.5; Trident/5.0; IEMobile/9.0; NOKIA; Lumia 800)"
"user_agent":"Mozilla/5.0 (iPad; CPU OS 9_0_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13A404 Safari/601.1"
"user_agent":"Mozilla/5.0 (iPad; CPU OS 9_0_2 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Mobile/13A452 [FBAN/FBIOS***]"
"user_agent":"Mozilla/5.0 (iPad; CPU OS 9_0_2 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13A452 Safari/601.1"
"user_agent":"Mozilla/5.0 (iPad; CPU OS 9_0_2 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13A452 Safari/601.1"
"user_agent":"Mozilla/5.0 (iPad; CPU OS 9_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13B143 Safari/601.1"
"user_agent":"Mozilla/5.0 (iPad; CPU OS 9_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13B143 Safari/601.1"
"user_agent":"Mozilla/5.0 (iPad; CPU OS 9_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13B143 Safari/601.1"
"user_agent":"Mozilla/5.0 (iPhone; CPU iPhone OS 9_0_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Mobile/13A405 YJApp-IOS jp.co.yahoo.ipn.appli/4.3.1"
"user_agent":"Mozilla/5.0 (iPhone; CPU iPhone OS 9_0_2 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13A452 Safari/601.1"
"user_agent":"Mozilla/5.0 (iPhone; CPU iPhone OS 9_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13B143 Safari/601.1"
"user_agent":"Mozilla/5.0 (iPhone; CPU iPhone OS 9_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13B143 Safari/601.1"
"user_agent":"Mozilla/5.0 (iPhone; CPU iPhone OS 9_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13B143 Safari/601.1"
"user_agent":"Mozilla/5.0 (iPhone; CPU iPhone OS 9_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13B143 Safari/601.1"
"user_agent":"Mozilla/5.0 (Linux; U; Android 4.4.4; en-us; Lenovo A6000 Build/KTU84P) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 UCBrowser/9.7.5.418 U3/0.8.0 Mobile Safari/533.1"
"user_agent":"Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.80 Safari/537.36"
```
Mostly:
* IE 10 (Windows 7; NT 6.1)
* IE 9 (Windows 7; NT 6.1)
* Mobile Safari 9 (iOS 9)
The majority was from Mobile Safari from iOS 9, which was released last month and (re-)introduced Navigation Timing support. Perhaps their implementation wasn't standards compliant?
The presence of IE9 and IE10 values is also noteworthy. We should look into our traffic stats and see how this total of 5-10% relates to their total traffic. Perhaps this is all IE9/10/iOS 9 clients? Or just some? Note that Navigation Timing is sampled. In WMF production it is sampled 1:1000.