Background
Absolute : The browser will always interpret / as the root of the hostname. For example, if my site was http://google.com/ and I specified /css/images.css , then it will look for it in http://google.com/css/images.css . If your project root was actually in /myproject/ , it would not find the css file. Therefore, you need to determine where the root directory of your project refers to the host name, and specify it in the href notation.
Relative . If you want to link to what you know, this is the same path to the url - that is, if it is in the same folder, for example http://mysite.com/myUrlPath/index.html and http://mysite.com/myUrlPath/css/style.css , and you know that there will always be this way , you can go against the convention and specify a relative path without placing a leading / in front of your path, for example, css/style.css .
File system notation . Alternatively, you can use standard file systems such as .. If you follow http://google.com/images/../images/../images/myImage.png , it will be the same as http://google.com/images/myImage.png . If you want to reference what is in the same directory from your file, use ../myFile.css .
Your particular case
In your case, you have two options:
<link rel="stylesheet" type="text/css" href="/ServletApp/css/styles.css"/><link rel="stylesheet" type="text/css" href="css/styles.css"/>
The first will be more specific and compatible if you move things, however, if you plan to save the file in the same place , and you plan to delete the / ServletApp / part of the URL , then the second solution is better.