The problem of two requests when the src attribute value of the img tag is empty (non-IE browser)

When the img src value is empty, two requests are made. Some students may have encountered similar situations before. When there is a tag like <img src=''>, an extra request will be made to the page.
Based on the analysis of an article, I guess that all tags that actively introduce external files will have this problem, such as embed, link, etc.
Therefore, it was verified by experiment.
The following are preliminary conclusions :

Tags/Requests/Browser	IE	FF, Safari, Chrome
Img	2	2
Embed	1	2
Background: url	1	2
Link	1	2
Input type='image'	2	2
Script	1	2
bgsound	2	—

Note :
1. For the two requests of IE and Firefox, the pages they request are different.
IE: The request is for the directory where the accessed file is located, such as the img tag, the second request is http://www.xxxx.cn/test/
FF, Safari, Chrome: The requested file is the one being accessed

2. For the embed tag, the above description is when type is not empty. When type is empty, FF requests 1, and IE responds differently depending on the type value.

3. If the above tags do not contain src="", no secondary request will occur.

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