The href in your link element specifes a URL-relative path that will only work if the css file is in the same folder as the JSP
and if the JSP is directly addressed. Since in most web apps neither of these are the case, you need to use server-relative addressing for the files.
Let's say that the css files is in a folder named styles at the context root. Adressing it would be:
<link href="${pageContext.request.contextPath}/styles/mycss.css" ...
(assuming JSP 2.0)