I believe the browser is looking for those assets FROM the root of the webserver. This is difficult because it is easy to start developing on your machine WITHOUT actually using a webserver ( just by loading local files through your browser)
You could start by packaging your html and css/js together?
a directory structure something like:
-yourapp
- index.html
- assets
- css
- js
- myPage.js
Then your script tag (from index.html) could look like
<script src="assets/js/myPage.js"></script>
An added benifit of packaging your html and assets in one directory is that you can copy the directory and give it to someone else or put it on another machine and it will work great.