Dompdf / libxml 2.9.5 issue on new GAE SE PHP 5.5 build

[Geoffrey Arnold]

Heads-up Google App Engine PHP Standard Environment Users,

The latest build of GAE PHP 5.5 (Build Date: Sep 28 2017 10:04:45) can break Dompdf with the error "No block-level parent found.  Not good." 

It's related to a bug in libxml 2.9.5 and a workaround (enable the HTML5 parser) is detailed here:

https://github.com/dompdf/dompdf/issues/1494

The prior version of GAE PHP 5.5 was built with libxml 2.9.4 which did not have this issue.

This caused a minor panic over here so I hope this saves someone the scare,

Geoff.