If you check your browser's developer console, you'll see lots of 400 errors on the page without the trailing slash. An example URL: http://fitsoft.pythonanywhere.com/css/responsive.css. Looking at what happens when you hit the page with the trailing slash, I think the URL that it should be looking at is http://fitsoft.pythonanywhere.com/template/css/responsive.css.
This is because you're including the stylesheets into your HTML using code like this:
<link rel="stylesheet" href="css/responsive.css">
The way URLs are calculated by the browser, based on the document URL is:
- If the document URL ends with a slash, it's treated like a "directory", and so relative URLs are just added on to the end. So,
- If the document URL does not end with a slash, it's treated like a "file", and so relative URLs are based on its containing "directory". Thus:
If you want your page to be able to get its CSS regardless of whether it's accessed using a slash or not, you'll need to use an absolute URL to reference the assets, like this:
<link rel="stylesheet" href="/templates/css/responsive.css">