Joomla users with fair knowledge of HTML know that Joomla editors (such as TinyMCE) sometimes can cut off a part(s) of their HTML code. How to fix this? There are different solutions for Joomla 1.5 and newer Joomla 1.6 and 1.7.
You can fix this if you disable WYSIWYG editor in Global Configuration. To do that, go to Site > Global Configuration and for Default WYSIWYG Editor select "Editor - No Editor". Then get back to edit your article (or Custom HTML module) and try to re-enter your code and save again. Parts of your HTML shall not be cut off this time.
Joomla 1.6, 1.7 and later
Even if you disable WYSIWYG editor, parts of your HTML code can still be cut off. That's because since Joomla 1.6 there are new Filtering functions added. They cut off all "unsafe" code. If you decide to disable this feature, you can if you go to Content > Article Manager > Options (icon in top right corner). Then select the Text Filters tab. We guess you are editing site as Super User, so select "No Filtering" option in the Super Users row. Your HTML code will not be cut of after this (if you are editing site as Super User), no matter what WYSIWYG editor you are using.
In this version of Joomla, text filtering options looks the same as on previous version, but they are now moved to the new place. You can access them in Site > Global Configuration > Text Filters.