Прогоняем через http://validator.w3.org по очереди:
The document located at <http://www.opera.com/>; was checked and found to be valid XHTML 1.0 Strict.
The document located at <http://www.konqueror.org/>; was checked and found to be valid XHTML 1.0 Transitional.
The document located at <http://www.mozilla.org/>; was checked and found to be valid HTML 4.01 Strict.
<http://www.microsoft.com/windows/ie/default.mspx>; - "This page is not Valid HTML 4.01 Transitional!" (41 ошибка)
<http://www.apple.com/safari/>; - "This page is not Valid HTML 4.01 Transitional!" (58 ошибок)
Делаем выводы =)
