Joomla allows you to specify which HTML tags you can use in your articles.

You can configure this setting from your Joomla admin area > Article Manager -> Parameters. There you will see a section named Filter type. It allows you to choose one of three options:

• Blacklist (Default) - you can specify which HTML tags should be filtered.
• Whitelist - you can specify which HTML tags should be allowed in your articles, all others will be filtered. 
• No html - no HTML allowed.

To whitelist or blacklist an HTML tag (depending on which of the above options you have selected), you should enter the tag in the Filter tags field. You can whitelist/blacklist HTML tag attributes by listing them in the Filter attributes field.

The default Blacklist filters the following tags:

'applet', 'body', 'bgsound', 'base', 'basefont', 'embed', 'frame', 'frameset', 'head', 'html', 'id', 'iframe', 'ilayer', 'layer', 'link', 'meta', 'name', 'object', 'script', 'style', 'title', 'xml'

and the following attributes:

'action', 'background', 'codebase', 'dynsrc', 'lowsrc'