前段时间有项目是在HTTPS协议下引入了HTTP协议的图，结果导致图不能显示，提示如下：
Mixed Content: The page at 'https://www.xxx.cn' was loaded over HTTPS, but requested an insecure element 'http://www.xxx.cn/assets/t.png' This request was automatically upgraded to HTTPS, For more information see'https://blog.chromium.org/2019/10/no-more-mixed- messages-about-https.html'
通过提示可以看到，HTTPS协议下引入HTTP协议的图，是认为不安全的，图的地址会被自动转换为HTTPS协议，因此会导致找不到图
其实上述情况，css，js，都会有该限制。
原则上HTTPS调用HTTP资源受到限制，HTTP调用HTTPS资源不受限制
