The W3C guidelines are so that all browsers display content the same way. There is a free tool on the w3c site that will validate your html against standards.
I'm designing a professional site at the moment and every single page is html4.01 compliant. i.e. the site looks the same in IE opera and Netscape.
It's down to the web page author whether they want their sites to display correctly.
To be fair though on the stats I have, 95% of people use IE. 4% netscape, 1% everything else.
I like opera myself because of the pop up window blocking but have real problems with compatability sometimes.
If it ain't broke - try harder.
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