When the users create media items and media folders in Sitecore that contain white space all media URLs will also contain white spaces. This is apparently frowned upon by W3C and their html-validator.
One would think that the normal <encodeNameReplacements> in web.config also handles media URLs, but it does not. It is however realtively simple to override the GetMediaUrl(…) that Sitecore uses without being too intrusive. Continue reading Replace white space in media URLs in Sitecore