web中ajax跨域与同源文章 from 阮一峰

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