The problem here is that the width is a size of the inner area with text, and the padding with border are "wrapped" around it. That specification makes no sense, but most modern browsers follow it.
Your savior is called box-sizing: border-box;.
.threadbit .thread {
/* ... some stuff ... */
-webkit-box-sizing: border-box; /* Safari/Chrome, other WebKit */
-moz-box-sizing: border-box; /* Firefox, other Gecko */
box-sizing: border-box; /* Opera/IE 8+ */
}
Look here: jsFiddle
More info on this property here and here.
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