Why $(window).on('load', function(){}) waits for no download the background-image?

Hello! Today to debug some parameter, using alert() on the LAN and noticed that the elements which have background, or not loading the background, or the very top. And appeared only after closing the popup alert window. Ie a script is triggered earlier, though I thought that this method is waiting for a full download of all resources.

1. As it should be?
2. Why accounting method $(window).load does not work, gives an error?
(Uncaught TypeError: e.indexOf is not a function
at k.fn.init.k.fn.load (jquery 3.4.1.min.js:2)
at index.html:83)
March 23rd 20 at 19:40
1 answer
March 23rd 20 at 19:42
In *.html move the connection script for connecting css
<link ...>
<script ...>

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