Don't keep your CSS or JavaScript files inside the WEB-INF directory. According to this StackOverflow thread, "WEB-INF resources are accessible to the resource loader of your Web-Application and not directly visible to the public".
Instead of that create your project files structure like below,

Later your can use/refer your CSS files like below.
<!DOCTYPE html>
<html >
<head>
<meta charset="UTF-8">
<title>Tennis</title>
<link rel="stylesheet" type="text/css" href="css/style.css">
</head>
<body>
...
</body>
</html>
Answer from Shashanth on Stack OverflowVideos
best to shoot it for the help of PHP but in the opposite way:
pgup.php
<!DOCTYPE HTML>
<html lang="de">
<head>
<link rel="stylesheet" type="text/css" href="../css/main.css" />
</head>
<body>
index.php
<?php include('pgup.php'); ?>
<div id="content" class="content">
Hello
</div>
<?php include('pgdown.php'); ?>
pgdown.php
</body>
</html>
main.css
/*(for id)*/
#content{
}
/*(or for class)*/
.content{
}
Class content not found :
**This means, the class named "content" is not defined or not found in HTML/CSS file(s).**
So, either create a new CSS file and make a reference from the html page or use tag to define the CSS definition in the body.html itself.

Browsers do not have direct access to resources under WEB-INF; if you really need it there (which would be weird) you'll need to stream it back yourself.
Otherwise, move it out of there, include it as a regular CSS resource, but generate the file's path using or the Spring equivalent.
you can use like this inside jsp
<link rel="stylesheet" type="text/css" href="${pageContext.request.contextPath}/source/css/reset.css">
You cannot place a css file outside the pages folder simply because everything exept that folder is not accessible by the Client. Dont forget that a css file is included like this:
<link rel="stylesheet" type="text/css" href="css/notification.css" />
You dont want someone to change it to <link rel="stylesheet" type="text/css" href="../src/MyPreciousJavaFile.java" /> and simply get the coding of any other file.
src/main/resources- is the directory of properties file or xml file that used by Java classes. For css you should place your file in webapp directory like src/main/webapp/resources. Client can't get any access outside of webapp directory.