Complete list of reasons why a css file might not be working

Question

This problem is making me feel like an absolute noob.

Here is the head of my .html file:

Answer 1

I was facing the same problem, but the reason was the styling in the css is wrapped in an ID that is not exist

#navi ul{
    color: red;
}

the #navi selector is nowhere in the HTML because i forgot to add it before writing the CSS. in my case I worked with Less file so I nested it like this

#navi{
    ul{
       color:red;
   }
}

I hope this help to re-check when things doesn't work