Browser check and correct the loaded html code during the whole page update. However, this does not happen during the direct DOM update (Ajax request/response). So, this might cause the content broken, disappearance if the code is not standard compliance.
Included into the RichFaces parsers check and correct the code. i.e. prevent the possible glitches. You can set the option to NONE. Just take the whole responsibility for the code you write.
Read more about the possible options in the Developers Guide (Filter Configuration section)