How do I use Access-Control-Allow-Origin? Does it just go in between the html head tags?
There are 3 ways to allow cross domain origin (excluding jsonp):
1) Set the header in the page directly using a templating language like PHP. Keep in mind there can be no HTML before your header or it will fail.
<?php header("Access-Control-Allow-Origin: http://example.com"); ?>
2) Modify the server configuration file (apache.conf) and add this line. Note that "*" represents allow all. Some systems might also need the credential set. In general allow all access is a security risk and should be avoided:
Header set Access-Control-Allow-Origin "*"
Header set Access-Control-Allow-Credentials true
3) To allow multiple domains on Apache web servers add the following to your config file
<IfModule mod_headers.c>
SetEnvIf Origin "http(s)?://(www\.)?(example.org|example.com)$" AccessControlAllowOrigin=$0$1
Header add Access-Control-Allow-Origin %{AccessControlAllowOrigin}e env=AccessControlAllowOrigin
Header set Access-Control-Allow-Credentials true
</IfModule>
4) For development use only hack your browser and allow unlimited CORS using the Chrome Allow-Control-Allow-Origin extension
5) Disable CORS in Chrome: Quit Chrome completely. Open a terminal and execute the following. Just be cautious you are disabling web security:
open -a Google\ Chrome --args --disable-web-security --user-data-dir
That is an HTTP header. You would configure your webserver or webapp to send this header ideally. Perhaps in htaccess or PHP.
Alternatively you might be able to use
<head>...<meta http-equiv="Access-Control-Allow-Origin" content="*">...</head>
I do not know if that would work. Not all HTTP headers can be configured directly in the HTML.
This works as an alternative to many HTTP headers, but see @EricLaw's comment below. This particular header is different.
Caveat
This answer is strictly about how to set headers. I do not know anything about allowing cross domain requests.
About HTTP Headers
Every request and response has headers. The browser sends this to the webserver
GET /index.htm HTTP/1.1
Then the headers
Host: www.example.com
User-Agent: (Browser/OS name and version information)
.. Additional headers indicating supported compression types and content types and other info
Then the server sends a response
Content-type: text/html
Content-length: (number of bytes in file (optional))
Date: (server clock)
Server: (Webserver name and version information)
Additional headers can be configured for example Cache-Control, it all depends on your language (PHP, CGI, Java, htaccess) and webserver (Apache, etc).