PHP throws warning for missing LIBXML_HTML_NODEFDTD constant

[schlessera]

Bug Description

From https://wordpress.org/support/topic/php-warning-use-of-undefined-constant-libxml_html_nodefdtd-assumed-libxml_h/ :

[31-Mar-2020 06:49:41 UTC] PHP Warning: Use of undefined constant LIBXML_HTML_NODEFDTD – assumed ‘LIBXML_HTML_NODEFDTD’ (this will throw an Error in a future version of PHP) in /home/xxx/public_html/wp-content/plugins/amp/vendor/ampproject/common/src/Dom/Document.php on line 393
[31-Mar-2020 06:49:41 UTC] PHP Warning: A non-numeric value encountered in /home/xxx/public_html/wp-content/plugins/amp/vendor/ampproject/common/src/Dom/Document.php on line 393

Expected Behaviour

No warnings should be thrown on supported PHP versions.

Steps to reproduce

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Screenshots

Additional context

• WordPress version:
• Plugin version:
• Gutenberg plugin version (if applicable):
• AMP plugin template mode:
• PHP version:
• OS:
• Browser: [e.g. chrome, safari]
• Device: [e.g. iPhone6]

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