Disabled automatic content filtering.
- go to Website & Content → Browse Website
- click on Content Mode tab
- click on Edit Content button
- in the opened Content Editor window press Source button
- in the Content Editor window's textarea input test content <div class="bla-bla-bla">test</div>
- in the Content Editor window press Source button twice - to switch to WYSIWYG mode and then back to HTML source mode
- confirm, that test content is not changed, class="bla-bla-bla" attribute was not deleted automatically by Content Filtering subsystem