I am trying to add custom CSS in the WYSIWYG editor using the ‘source code’ mode, but when I switch back to visual mode, my changes are not sticking. Can somebody help?
The question is not really an ACF question but a WP/TniyMCE question.
Here is a good article: https://vip.wordpress.com/documentation/register-additional-html-attributes-for-tinymce-and-wp-kses/
you can find more info by doing a google search for “altering allowed html elements in wp tinymce”