Re: [whatwg/url] Define hosts' public suffix and registrable domain. (#391)

sleevi commented on this pull request.



> @@ -272,6 +272,94 @@ for further processing.
 U+0020 SPACE, U+0023 (#), U+0025 (%), U+002F (/), U+003A (:), U+003F (?), U+0040 (@), U+005B ([),
 U+005C (\), or U+005D (]).
 
+<p>A <a for=/>host</a>'s <dfn for=host export>public suffix</dfn> is the portion of a
+<a for=/>host</a> which is controlled by a registrar, public or otherwise. To obtain
+<var>host</var>'s <a for=host>public suffix</a>, run these steps:
+
+<ol>
+ <li><p>If <var>host</var> is not a <a>domain</a>, then return null.
+
+ <li><p>Return the <a for=host>public suffix</a> obtained by executing the
+ <a href="https://publicsuffix.org/list/">algorithm</a> defined by the Public Suffix List on
+ <var>host</var>. [[!PSL]].
+</ol>
+
+<p>A <a for=/>host</a>'s <dfn for=host export>registrable domain</dfn> is a <a>domain</a> that could
+be registered at a registry. To obtain <var>host</var>'s <a for=host>registrable domain</a>, run

So a given host may have multiple public suffixes expressed within it.

Perhaps:
The domain formed by the most specific public suffix for host, along with the domain label immediately preceding it?

>From a spec question, what do you want this definition to entail for the `appspot.com` case?

That is,
- `foo.bar.appspot.com` is "obviously" going to return `bar.appspot.com` as the registerable domain (with `appspot.com` as the public suffix), and the same would be expected if just `bar.appspot.com`.
- What do you expect this machinery to return for `appspot.com`? `appspot.com` is on the PSL, so that is a public suffix, but `appspot.com` is also a registerable domain under the `com` PSL.

I seem to recall that different platform features interpret that differently (navigation vs cookies, for example)

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