Why is CKEditor 4 changing my code?

CKEditor 4 is a standards-compliant (W3C) HTML editor which means it corrects code if it doesn’t follow current web standards. What’s more, some browsers may have their own interpretation of HTML.

Also, CKEditor 4 is featured with Advanced Content Filter (ACF) which transforms and removes disallowed elements. If you paste content into CKEditor 4 and notice that some elements are removed, then chances are high that it was removed by ACF. For instance, if your toolbar doesn’t include the Bold button, CKEditor 4 will strip bold tags from the source code.

To learn how to configure or disable ACF, go to Content Filtering article. See also an online demo of how ACF works in CKEditor 4 Examples.