I’ve run my css through a validator (nothing is wrong)
My header looks as following (with irrelevant pieces removed)
<head>
<link href="https://fonts.googleapis.com/css2?family=Lato:wght@400&display=swap" rel="Stylesheet">
<title>Projekter</title>
<link rel="Stylesheet" href="Stylesheet.css">
</head>
and my stylesheet is named Stylesheet.css in the following folder hierachy

What could possibly be wrong here?
Please do tell me if I’m missing giving some relevant information.
>Solution :
The stylesheet call path is invalid.
The index.html file is in the HTML folder and Stylesheet.css is outside the HTML folder.
Please correct it as follows.
<link rel="stylesheet" href="../Stylesheet.css">
Chrome DevTools makes it easier to determine the cause.