<...> proprietary attribute "..."

Cause:

The tag contains an attribute that is proprietary to Netscape or Internet Explorer.

Solution:

Most tags have a equivalent in standard HTML or CSS.

Tag Proprietary attribute Standard attribute
<body> marginwidth=0
marginheight=0
leftmargin=0
topmargin=0
style="margin: 0"
<frameset> border=0
frameborder=0
framespacing=0
None with frameset. See here.
Use a HTML table with iframe for the same result.
<table> height=100% style="height: 100%"
<table> nowrap style="white-space: nowrap"
or use nowrap in <td> or <th>
<td> background="abc.gif" style="background-image:url(abc.gif)"

Sample: <table> proprietary attribute "height"

BAD    <table height="100%">
GOOD   <table style="height: 100%">

References:

W3C attributes http://www.w3.org/TR/html4/index/attributes.html