跨域资源共享 CORS 详解
http://www.ruanyifeng.com/blog/2016/04/cors.html //这个是最明白的。。建议看看。
http://www.ruanyifeng.com/blog/2016/04/same-origin-policy.html
http://www.ruanyifeng.com/blog/2016/04/cors.html
http://javascript.ruanyifeng.com/bom/ajax.html
//相当详细, 完备
https://en.wikipedia.org/wiki/Cross-origin_resource_sharing