width: 100% doesn't play well with margins

[taralx]

https://webrtchacks.com/sdp-anatomy/

There's a horizontal scrollbar because width is mixed with margin in two places:

1. <div id="container" class="contain" style="width:100%; margin:10px;">
2. #description has margins, but inherits width.

Both of these should be paddings, which gets rid of the annoying horizontal scrollbar.