In theory, this is specified in section 3.1 of <a href="https:&#x2F;&#x2F;www.ietf.org&#x2F;rfc&#x2F;rfc3987.txt" rel="nofollow">https:&#x2F;&#x2F;www.ietf.org&#x2F;rfc&#x2F;rfc3987.txt</a> (including the &quot;MAY&quot; section beginning &quot;Systems accepting IRIs MAY convert the ireg-name component of an IRI as follows&quot;). That covers the distinction between the domain and the rest of the URL.<p>The use of the page encoding is part of the browser behavior, not any RFC. It has been retroactively specified in <a href="https:&#x2F;&#x2F;url.spec.whatwg.org&#x2F;#url-parsing" rel="nofollow">https:&#x2F;&#x2F;url.spec.whatwg.org&#x2F;#url-parsing</a>