lib/internal/Magento/Framework/View/Element/Messages.php#L263-L291
/**
* Render messages in HTML format grouped by type
*
* @return string
*/
protected function _renderMessagesByType()
{
$html = '';
foreach ($this->getMessageTypes() as $type) {
if ($messages = $this->getMessagesByType($type)) {
if (!$html) {
$html .= '<' . $this->firstLevelTagName . ' class="messages">';
}
foreach ($messages as $message) {
$html .= '<' . $this->secondLevelTagName . ' class="message ' . 'message-' . $type . ' ' . $type .
'">';
$html .= '<' . $this->contentWrapTagName . $this->getUiId('message', $type) . '>';
$html .= $this->interpretationStrategy->interpret($message);
$html .= '</' . $this->contentWrapTagName . '>';
$html .= '</' . $this->secondLevelTagName . '>';
}
}
}
if ($html) {
$html .= '</' . $this->firstLevelTagName . '>';
}
return $html;
}


