To connect javascript file with html,
we use, src="" attribute.
Example:
<script type="text/javascript" src="something.js"></script>
but in CSS we use, href="" attribute.
Example:
<link rel="stylesheet" type="text/css" href="something.css">
If I use href="" for javascript and src="" for CSS then,It’s not working!
But why like that? 
Hey @Plabon_Kumer_Sarker,
CSS uses href because it uses the <link> tag that means it links to something. HREF stands for Hypertext Reference. From the name we can attribute that it is referencing the file into HTML. It is linking the reference to it. While src just stands for source, which just embed the file into the HTML.
This also means that src can give the script 3 modes to run in, asynchronously, defer, and just runs where the script tag exist. This means we can run the script asynchronously with the HTML, or we can run the script after the page is loaded or just run it when the script tag is seen. This means the script tag can pause the parsing process of the HTML and wait until the Javascript is run or vice versa.
While the href tag tells the HTML parser that it’s just a reference link and it won’t stop the parser to parse the HTML while also parsing the CSS with the HTML. It might not render instantly, but it’s parsed and ready to go.
Hope this helps 
This explanation is gathered from: