To post HTML, videos and/or images, make sure you are using the "Visual Editor."
Along the top border of the "Post your reply to this topic" window is a message that indicates which editor is in use. The message toggles. If the message is, "switch to text editor," you are using the Visual Editor. If the message is, "switch to visual editor," you are using the Text Editor."
The Text Editor is used for messages that are "text only."
The Visual Editor is used for embedding objects (images-videos) and HTML (links).
To insert HTML into your message, click the "HTML" link at the right of the Visual Editor toolbar. The HTML Source Editor will appear. After inserting your HTML message, make sure you click the "Update" button, at the bottom of the HTML Text Editor window.