I +1'd Miro's answer for the link to the header-checker site http://www.webconfs.com/http-header-check.php. It pops up an obnoxious ad every time you use it, but it is, nevertheless, very useful for verifying the presence of the Access-Control-Allow-Origin header.
I'm reading a .json file from the javascript on my web page. I found that adding the following to my .htaccess file fixed the problem when viewing my web page in IE 11 (version 11.447.14393.0):
<FilesMatch "\.(json)$">
<IfModule mod_headers.c>
Header set Access-Control-Allow-Origin "*"
</IfModule>
</FilesMatch>
I also added the following to /etc/httpd.conf (Apache's configuration file):
AllowOverride All
The header-checker site verified that the Access-Control-Allow-Origin header is now being sent (thanks, Miro!).
However, Firefox 50.0.2, Opera 41.0.2353.69, and Edge 38.14393.0.0 all fetch the file anyhow, even without the Access-Control-Allow-Origin header. (Note: they might be checking IP addresses, since the two domains I was using are both hosted on the same server, at the same IPv4 address.)
However, Chrome 54.0.2840.99 m (64-bit) ignores the Access-Control-Allow-Origin header and fails anyhow, erroneously reporting:
No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin '{mydomain}' is therefore not allowed access.
I think this has got to be some sort of "first." IE is working correctly; Chrome, Firefox, Opera and Edge are all buggy; and Chrome is the worst. Isn't that the exact opposite of the usual case?