The link seems to be OK, I can see the styles when I open the link. But I do not see the style from file applied in the html element. In the file there is black background for the div, but it stays without background.
Are you expecting the style to be applied only to the div contained in it? In effect to be scoped? To do that you would need a scoped attribute, except that support has been removed from recent Firefox versions. Since the scoped attribute is no longer supported, the style element is now strictly metadata content and should not contain other elements.
For further investigations use DOM inspector or the web console inspector to see what CSS rules are actually being applied to the div. Then you can find out if your style is even being recognised as a CSS rule and whether it is being overridden by another rule.
I have added red arrows pointing to the text which should be applied from the css file (Font-Family, Font-Size, color:white). The div’s background must be completly black so the text is visible.
Now it works very well. I’m thankful to solve it because last days I had many bugs after I did some major updates. Now at least few more problems solved.