renderJS gadget HTML uses relative URLs to link to there javascript/CSS files.
Absolute URL calculation is resolved dynamically by the browser when loading the gadget.

When accessing an ERP5 web section without a trailing / in the URL, the browser will calculate absolute URL from the parent document and not the web site itself.

Example:
In http://foo.com/web_site_module/bar , the relative URL couscous.js will be resolved http://foo.com/web_site_module/couscous.js
If couscous.js is a document from DMS (Web Page for example), such URL can not be resolved and leads to a 404 error.

One solution to solve this is to redirect (302) the browser when accessing a Web Section (Web Site is a web section) directly in ERP5.
Example:
http://foo.com/web_site_module/bar ->  http://foo.com/web_site_module/bar/
But  http://foo.com/web_site_module/bar/view should not redirect