Path to folder outside of folder
If I am understanding, your directory structure is as follows:
siteroot
main
math
css
You are looking to link to a style sheet in /main/css from /main/math/index.html.
There are two solutions, the easiest is to use an absolute path:
<link rel="stylesheet" href="/main/css/basic.css" type="text/css" media="screen" />
Absolute paths are a better solution and will cause less headache down the road. I do not know of any drawbacks, either.
Alternatively, you could use '..' to traverse up a directory
<link rel="stylesheet" href="../css/basic.css" type="text/css" media="screen" />
For example your directory is like this:
Desktop >
ProjectFolder >
index.html
css >
style.css
images >
img.png
You are at your style.css and you want to use img.png as a background-image, use this:
url("../images/img.png")
Works for me!