I have installed plugin news. It is generating such html:
<div id="c69" class="csc-default">
You can disable using TypoScript
Put above line in your Template > root template > Setup
It'll give you clean HTML output as your layout without default wrap of
css_styled_content. or if you see classes with extension related then must be from extension which you used. In extBase structure you'll find ext_key/Resources/Private/Layouts and Templates.