# go to {nav 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