... proprietary attribute value "..."

Cause:

The value used in the attribute is a proprietary one from Netscape or Internet Explorer.

Sample:

<img> proprietary attribute value "absmiddle"

See : http://www.w3.org/TR/html4/struct/objects.html#adef-align-IMG

BAD      <p><img src="hello.gif" alt="hello" align="absmiddle">hello</p>
GOOD  ** <p><img src="hello.gif" alt="hello" align="middle"> hello</p>
GOOD  ** <p><img src="hello.gif" alt="hello" style="vertical-align: middle;"> hello</p>
GOOD but COMPLEX

<style type="text/css" media="all">
* {padding: 0; margin: 0;}
h1, p{margin: 0.5em;}
p.flag img, span {vertical-align: middle;display: inline-block;}
p.flag span {padding-left: 5px;}
</style>
...
<p class="flag"><img src="hello.gif" alt="hello"><span>hello</span></p>
...

** Absmiddle and middle do not have exactly the same behavior.
> "absmiddle" align the center of the image with the vertical center of the text next to it.

> "middle" align the center of the image with the bottom of the text next to it.

References:

HTML specification: http://www.w3.org/TR/html4/