Laravel – Why Element 'iframe' is not supported for youtube links with mews/purifier?

The configuration looks right for HTML Purifier. This script does what it should:

// requiring HTMLPurifier.auto.php up here somewhere, no other code up here but that.

$dirtyHtml = '
<iframe width="560" height="315" src="https://www.youtube.com/embed/ydYDqZQpim8" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>
';

$knobs = [
  'AutoFormat.AutoParagraph' => false,
  'AutoFormat.RemoveEmpty' => true,
  'CSS.AllowedProperties' => 'font,font-size,font-weight,font-style,font-family,text-decoration,padding-left,color,background-color,text-align',
  'Filter.YouTube' => 'true',
  'HTML.Allowed' => 'iframe[src|width|height|class|frameborder],h1,h2,h3,div,blockquote,table,tbody,tr,td,figure,b,strong,i,em,u,a[href|title],ul,ol,li,p[style],br,span[style],img[width|height|alt|src]',
  'HTML.Doctype' => 'HTML 4.01 Transitional',
  'HTML.SafeIframe' => 'true',
  'URI.SafeIframeRegexp' => '%.+%',
];

$config = HTMLPurifier_Config::createDefault();
foreach ($knobs as $setting => $value) {
  $config->set($setting, $value);
}
$purifier = new HTMLPurifier($config);

$cleanHtml = $purifier->purify($dirtyHtml);

echo $cleanHtml . PHP_EOL;

This gives me:

<iframe width="560" height="315" src="https://www.youtube.com/embed/ydYDqZQpim8" frameborder="0"></iframe>

If I comment out the lines that are in your 'youtube' configuration block, however, like so:

// ...
$knobs = [
  'AutoFormat.AutoParagraph' => false,
  'AutoFormat.RemoveEmpty' => true,
  'CSS.AllowedProperties' => 'font,font-size,font-weight,font-style,font-family,text-decoration,padding-left,color,background-color,text-align',
  // 'Filter.YouTube' => 'true',
  'HTML.Allowed' => 'iframe[src|width|height|class|frameborder],h1,h2,h3,div,blockquote,table,tbody,tr,td,figure,b,strong,i,em,u,a[href|title],ul,ol,li,p[style],br,span[style],img[width|height|alt|src]',
  'HTML.Doctype' => 'HTML 4.01 Transitional',
  // 'HTML.SafeIframe' => 'true',
  // 'URI.SafeIframeRegexp' => '%.+%',
];
// ...

…then I get your error:

PHP Warning:  Element 'iframe' is not supported (for information on implementing this, see the support forums)  in /<...>/HTMLDefinition.php on line 311
PHP Stack trace:
<...>

Unfortunately, I don’t know anything about the Laravel integration here, and how you choose which configuration is active, but if you make sure that the 'youtube' and 'default' blocks are actually being combined and used, that should solve your problem.