Why CKEditor change original element and class

Question

for example, I have following content will be used:

<div class="h21copy">
    <p>123</p>
</div>

When I initialize with CKEditor, it will become to:

<p>123</p>

screenshots blow:

Answer 1

paste in your config.js

config.allowedContent = true;

Further reading: https://ckeditor.com/docs/ckeditor4/latest/guide/dev_allowed_content_rules.html