Custom HTML modules and articles allow us to display custom HTML code inside any module position or inside an article. The only thing that you must do is to select a shortcode and copy - paste it into a module or an article.
During this process a problem may arise when you save the module or article: Most editors will strip the HTML tags and this will break the module or article.
The best way to create a custom module or an article is to use the CodeMirror editor which is already installed by default in your Joomla website. To change the editor, go to Joomla Administrator area > System > Global Configuration and in the field Default Editor select Editor - CodeMirror.
Note that even the most popular editors in Joomla like JCE could cause many problems in custom content modules unless they are properly set.