属性値の大文字・小文字区別

box
Box
BOX

DOCTYPE

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

X/HTML

<div id="box">box</div>
<div id="Box">Box</div>
<div id="BOX">BOX</div>

CSS

div#box {
  color: blue;
}

div#Box {
  color: gray;
}

div#BOX {
  color: yellow;
}
Taichi Kaminogoya.