HTML Purifier Config File
Something like this in a .json file should work:
{
  &quot;HTML.SafeIframe&quot;: true,
  &quot;URI.SafeIframeRegexp&quot;: &quot;%^(https?:)?//(www\\.youtube(?:-nocookie)?\\.com/embed/)%&quot;,
  &quot;Attr.AllowedFrameTargets&quot;: [&quot;_blank&quot;],
  &quot;HTML.AllowedComments&quot;: [&quot;pagebreak&quot;]
}

Note that the first two lines were adapted from https://stackoverflow.com/a/12784081/684 and the last two are the default options that Craft uses if no custom config is set.
Probably also worth giving this a read: http://htmlpurifier.org/docs/enduser-youtube.html