I've been using web on a prim for some time, but recently needed to display a YouTube video on a prim. Of course, I wantedthe video and not all the comments and other stuff on the page. I knew that there was a way to do this, but forgot, and it took some searching to find the very simple information I needed.
Some people though it was necessary to edit the prim to "cut off" the parts not needed, and some people thought a script would be needed. No. This is how to display just the video on your prim. All you need to do, in step-by-step form:
Create a prim by right clicking, choosing create and left clicking. Texture it as you wish.
Whilst in the edit menu, choose the texture tab.
Click select face in the upper part of the edit menu, and select the face which is the one you want to display the video.
Click the plus button at the bottom of the texture page of the edit menu, to add media to the prim. The edit menu changes and there is an input bar at the top of the page with a preview screen beneath it.
Copy the URL of the video you want to screen into this input bar, but where the URL says watch?, add in an underscore and the word popup. Thus where you once had watch? you will now have watch_popup? This is all you have to do in order to have just the video pane appearing rather than the whole page from YouTube.
Click reset, apply, OK.
If you wish, you *can* use slice to cut off the border which still may appear above and below your video pane.