Two usages of iFrame tags in HTML

I have been working on a project recently - Budou Collection. In short, it is to collect the pictures you like to the Budou page. Many aspects of iframe are used in this process, which can be summarized as follows:
1. As a pop-up layer to cover the bottom

If you have ever made a black mask covering the entire page, and the user happens to be using IE6, and there happens to be a select element on the page, then you will have a headache (I won’t go into the principles here). We will find that the pop-up DIV cannot cover the select, so it is our iframe's turn to appear. The logic is as follows:

Put an iframe at the same level as the pop-up div
Ensure that the z-index of the iframe is smaller than the z-index of the pop-up div
Add window resize and scroll events to ensure that the iframe can cover the entire page

Part of the code

2. Writing cookies across domains

There are two domains, a.com and b.com. Under certain conditions, some of the functions of b will appear on page a. Sometimes when operating page a, you need to do some operations on the cookies of domain b.com to facilitate the user's next visit. We only need to add an iframe (dynamic or fixed) to page a, with the src attribute pointing to a proxy page of b, and perform cookie operations on this page.