NotAcceptableError: Occasional no supported media type in accept header errors

We’re on Phoenix 1.2.5 and Elixir 1.3.4.

Our appsignal errors show occasional NotAcceptableError/406 errors for client requests. In this instance, the request headers look like they should definitely work.

Our pipeline is configured as: plug :accepts, ["html"]

And the headers reported with the issue are:

req_headers.accept	text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
req_headers.accept-charset	ISO-8859-1,utf-8;q=0.7,*;q=0.7
req_headers.accept-encoding	gzip
req_headers.accept-language	en-us,en;q=0.5
req_headers.connection	upgrade
req_headers.user-agent	Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Firefox/52.0

If I use a browser plugin to set my client to those headers, it works perfectly.

Given that our site is 99.9% web/HTML, is it possible to specify that a pipeline will accept anything and consistently just return the HTML if the client asked for something strange?