Allow `data-*` attributes

I tried the following code with HTMLPurifier 4.13.0 to allow generic data-* attributes:

<?php

require_once('vendor/autoload.php');

$config = HTMLPurifier_Config::createDefault();
$config->set('HTML.Allowed', 'p, *[style|class|data-*], img[src]');
$def = $config->getHTMLDefinition(true);
$purifier = new HTMLPurifier($config);
print $purifier->purify('<p data-test="foo" data-test2="bar" data-test3="baz">test</p>');

This prints a warning:

PHP Warning: Global attribute '*' is not supported in any elements (for information on implementing this, see the support forums) in /var/www/htmlpurifier/vendor/ezyang/htmlpurifier/library/HTMLPurifier/HTMLDefinition.php on line 390

Since http://htmlpurifier.org/phorum/ is no longer working, and I did not find anything similar in the GitHub issues list, could anyone point me to some information what would be needed to make this work (or just tell me my configuration was wrong and it actually should work?).

Thanks!

Start by reproducing the configuration in the issue and inspect library/HTMLPurifier/HTMLDefinition.php around line 390, where the warning is emitted. Trace how HTML.Allowed parses global attributes, then verify that the example preserves data-test, data-test2, and data-test3 without the warning.