There are a small list of HTML code that is not allowed in WordPress due to security reasons.
List of HTML Code NOT Allowed in WordPress
<embed> | Defines a container for an external application or interactive content (a plug-in).
<frame> | Defines a window (a frame) in a frameset
<iframe> | Defines an inline frame
<form> | Defines an HTML form for user input
<input> | Defines an input control
<object> | Defines an embedded object
<textarea> | Defines a multiline input control (text area)
If you don’t see a code you were hoping to use, see my HTML Code Allowed in WordPress blog post.