This function allow to repair bad partial HTML (without DOCTYPE, html, head or body tags).

partial-html-repair.php

<?php
function fixHtml ($html)
{
    libxml_use_internal_errors(true);

    $DOM = new \DOMDocument;
    $DOM->recover = true;
    $DOM->preserveWhiteSpace = false;
    $DOM->substituteEntities = false;
    $DOM->loadHtml('<?xml encoding="UTF-8">'.$html, LIBXML_NOBLANKS | LIBXML_ERR_NONE);

    libxml_use_internal_errors(false);

    return preg_replace('~<(?:!DOCTYPE|/?(?:\?xml|html|head|body))[^>]*>\s*~i', '', $DOM->saveHTML());
}