Background color #asdf is probably not valid HTML. Internet Explorer 5 for Windows displays it identically to color #a0df00; it seems to treat non-hexadecimal characters as zero digits.
Verify on your own machine! Paste this into an HTML file:
<html>
<head>
<title>asdf</title>
</head>
<body bgcolor="#asdf" text="#000000">
<h1>this is bgcolor="#asdf"</h1>
<table>
<tr>
<td bgcolor="#000000"><h1>your browser does not support td bgcolors</h1></td>
</tr>
<tr>
<td bgcolor="#a0df00"><h1>this is bgcolor="#a0df00"</h1></td>
</tr>
</table>
</body>
</html>
This code can also be used to determine how any two
HTML colors look together on screen.