- Legacy EnlighterJS code cannot be transformed to blocks by the "Transform to Blocks" utility of the classic editor.
Caused By: gutenberg/8648 - Newly created codeblocks (via Gutenberg) mess up the HTML with unused attributes because of a validation issue.
Caused By: gutenberg/8532 / gutenberg/7604 
As integral part of the Enlighter WordPress Plugin (future)
- Create the plugin directory 
wp-content/plugins/enlighter-gutenberg - Copy 
enlighter-gutenberg.phpanddist/into this directory - Activate the plugin
 
Required Directory Structure
wp-content/
   | - plugins/
   |      | -enlighter-gutenberg/
   |      |       |- enlighter-gutenberg.php
   |      |       |- dist/
   |      |       |      |- enlighterjs.gutenberg.min.css 
   |      |       |      |- enlighterjs.gutenberg.min.js
EnlighterJS.Gutenberg is OpenSource and licensed under the Terms of Mozilla Public License 2.0. You're welcome to contribute
