HTML iframe, 获取iframe子页面中的元素(基于JavaScript)

参考： https://blog.csdn.net/houyanhua1/article/details/79702337?utm_medium=distribute.pc_relevant.none-task-blog-BlogCommendFromMachineLearnPai2-1.channel_param&depth_1-utm_source=distribute.pc_relevant.none-task-blog-BlogCommendFromMachineLearnPai2-1.channel_param