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It is the CSS class that generated not compatible with Bootstrap CSS, I expected that I change the class by WYSIWYG editor, but the CSS in the generated HTML was replaced to Publii default, so I cannot utilize Bootstrap class.
This is the HTML I put in the WYSIWYG HTML editor
This is the generated HTML
<figure class="img-fluid"> </figure>
I cannot figure out how to fit Bootstrap CSS to the generated result.