[[RFC7595]]

<p>A <dfn export oldids=syntax-url-relative-with-fragment>relative-URL-with-fragment string</dfn>

must be a <a>relative-URL string</a>, optionally followed by U+0023 (#) and a

<a>URL-fragment string</a>.

must be:

<ul class=brief>

<li><p>the empty string, if <a>base URL</a> has an <a for=url>opaque path</a>;

<li><p>otherwise, a <a>relative-URL string</a>,

</ul>

<p>optionally followed by U+0023 (#) and a <a>URL-fragment string</a>.

<p>A <dfn export oldids=syntax-url-relative>relative-URL string</dfn> must be one of the following,

switching on <a>base URL</a>'s <a for=url>scheme</a>:

<p>any optionally followed by U+003F (?) and a <a>URL-query string</a>.

<p class=note>A non-null <a>base URL</a> is necessary when <a lt="URL parser">parsing</a> a

<a>relative-URL string</a>.

<a>relative-URL-with-fragment string</a>.

<p>A <dfn export>scheme-relative-special-URL string</dfn> must be "<code>//</code>", followed by a

<a>valid host string</a>, optionally followed by U+003A (:) and a <a>URL-port string</a>, optionally