Forcing Browser Into Quirks Mode

Gutenberg has trouble with the PHP metaboxes when the browser is in Quirks mode. See this thread here: https://github.com/WordPress/gutenberg/issues/11378

My site was having that problem because WP Rainbox Hilite was forcing my browser into Quirks Mode because of some scripts being loaded BEFORE the <DOCTYPE> declaration. I didn’t dig through the code, but here are the scripts that were being output while the plugin was active:

<script src="/cdn-cgi/apps/head/IRmukQd9xuplTY_UDzfOP0iBU2s.js"></script><script type="text/javascript"> var mce_wprainbow = {"l10n":{"code_language":"Code Language","line_numbers":"Line Numbers","code_properties":"Code Properties","starting_line":"Starting Line"},"languages":[{"text":"- None -","value":""},{"text":"Generic","value":"generic"},{"text":"CSS","value":"css"},{"text":"HTML","value":"html"},{"text":"JavaScript","value":"javascript"},{"text":"PHP","value":"php"}]};</script>

I’d love to keep using this plugin but it makes writing in Gutenberg difficult to impossible currently. Thanks!