[ketozhang]

To be fair, it's the OP that chose to use the URL with the extension. However, you could say WC3 could've disable their servers from automatically creating URLs with file extension if they wanted to follow this.

1. https://www.w3.org/Provider/Style/URI works

2. If https://www.w3.org/Provider/Style/URI.html is ever 404 then you get a very useful 300 page (e.g., try https://www.w3.org/Provider/Style/URI.foobar)

3. However, from (2) you can see Spanish page is encoded as `.../URL.html.es` which is bad because `.../URI.es` does not exist