Be aware that some HTML editors such as TinyMCE can cut off parts of your HTML code. In Joomla 2.5, parts of your HTML code can be cut off even if you have disabled the WYSIWYG editor. This is due to Joomla 1.6. There are new filtering functions that have been added. All “unsafe” code gets cut off. In order to fix this, go to your backed and select Site > Global Configuration > Text Filters.
Should you be editing the site as a Super User, select “No Filtering” in the Super Users row, regardless of the WYSIWYG editor being used.
If you’d like that other levels be allowed to enter HTML, simply remove them from the blacklist. Obviously you should exercise caution when removing users from the black list to enter HTML.
In similar other case, you’ll be able to use the default CodeMirror editor or any other 3rd party extension like RokPad.