Hello,
Please install and enable PHP Tidy extension to fix it. If you can't install Tidy, that is the recommended way, you'll need to edit the file lib/dompdfParser/include/parserhtml.cls.php and change the method _load_html; in this method you can remove the library htmlawed (used when Tidy is not available) and use your custom library to fix and repair HTML (or even remove htmlawed is the HTML and its CSS is correct).
But again, please install Tidy to work with HTML.
Regards.