<h3 id=idna>IDNA</h3>

<div algorithm>

<p>The <dfn id=concept-domain-to-ascii>domain to ASCII</dfn> algorithm, given a <a for=/>string</a>

<var>domain</var> and a boolean <var>beStrict</var>, runs these steps:

<p>The <dfn id=concept-domain-to-ascii>domain parser</dfn> algorithm, given a

<a for=/>scalar value string</a> <var>domain</var> and a boolean <var>beStrict</var>, runs these

steps. They return failure or a <a for=/>domain</a>.

<ol>

<li>

<div algorithm>

<p>The <dfn id=concept-domain-to-unicode>domain to Unicode</dfn> algorithm, given a <a>domain</a>

<var>domain</var> and a boolean <var>beStrict</var>, runs these steps:

<var>domain</var>, runs these steps:

<ol>

<li><p>Let <var>result</var> be the result of running

<a abstract-op lt=ToUnicode>Unicode ToUnicode</a> with <i>domain_name</i> set to <var>domain</var>,

<i>CheckHyphens</i> set to <var>beStrict</var>, <i>CheckBidi</i> set to true, <i>CheckJoiners</i>

set to true, <i>UseSTD3ASCIIRules</i> set to <var>beStrict</var>, <i>Transitional_Processing</i>

set to false, and <i>IgnoreInvalidPunycode</i> set to false. [[!UTS46]]

<i>CheckHyphens</i> set to false, <i>CheckBidi</i> set to true, <i>CheckJoiners</i> set to true,

<i>UseSTD3ASCIIRules</i> set to false, <i>Transitional_Processing</i> set to false, and

<i>IgnoreInvalidPunycode</i> set to false. [[!UTS46]]

<li>

<p>If an error was recorded, then return <var>domain</var>.

<p class=note>Because <var>domain</var> can only result from the <a>host parser</a>, any recorded

errors will already have been signified as <a>validation errors</a>. Returning <var>domain</var>

ensures <a>domain to ASCII</a> and <a>domain to Unicode</a> roundtrip on input such as

ensures the <a>domain parser</a> and <a>domain to Unicode</a> roundtrip on input such as

<code>xn--8i7caa</code>.

<li><p>Return <var>result</var>.

<p>A <a for=/>string</a> <var>input</var> is a <dfn>valid domain</dfn> if these steps return true:

<ol>

<li><p>Let <var>domain</var> be the result of running <a>domain to ASCII</a> with <var>input</var>

<li><p>Let <var>domain</var> be the result of running <a>domain parser</a> with <var>input</var>

and true.

<li><p>Return false if <var>domain</var> is failure; otherwise true.

<a for=string>percent-decoding</a> of <var>input</var>.

<p class=note>Alternatively <a>UTF-8 decode without BOM or fail</a> can be used, coupled with an

early return for failure, as <a>domain to ASCII</a> fails on U+FFFD (�).

early return for failure, as the <a>domain parser</a> fails on U+FFFD (�).

<li><p>Let <var>asciiDomain</var> be the result of running <a>domain to ASCII</a> with

<li><p>Let <var>asciiDomain</var> be the result of running <a>domain parser</a> with

<var>domain</var> and false.

<li><p>If <var>asciiDomain</var> is failure, then return failure.

<ul>

<li>

<p>Browsers should render a <a for=/>URL</a>'s <a for=url>host</a> by running

<a>domain to Unicode</a> with the <a for=/>URL</a>'s <a for=url>host</a> and false.

<p>If <a for=/>URL</a>'s <a for=url>host</a> is a <a for=/>domain</a>, browsers should render it

by running <a>domain to Unicode</a> with the <a for=/>URL</a>'s <a for=url>host</a>.

<p class=note>Various characters can be used in homograph spoofing attacks. Consider detecting

confusable characters and warning when they are in use. [[IDNFAQ]] [[UTS39]]