PNG versions of valid HTML images added

I added PNG versions of each of the valid HTML images:

    http://validator.w3.org/images/

and changed the HTML blurb given for successful validations to:

    <p>
      <a href="http://validator.w3.org/check/referer"><img border=0
         src="http://validator.w3.org/images/vh40"
         alt="Valid HTML 4.0!" height=31 width=88></a>
    </p>

so content negotiation will return PNG versions to browsers
that understand PNG, and GIFs otherwise.

I had to add "Options +MultiViews" to Apache's httpd.conf
to enable content negotiation, and I also uncommented the
"CacheNegotiatedDocs" line to allow proxies to cache
negotiated documents.

More info on Apache and content negotiation is here:

    http://www.apache.org/docs/content-negotiation.html

-- 
Gerald Oskoboiny       <gerald@w3.org>  +1 617 253 2920
System Administrator   http://www.w3.org/People/Gerald/
World Wide Web Consortium (W3C)      http://www.w3.org/