my HTML5 page starts with
<?php header("X-UA-Compatible: IE=9; IE=8; IE=7; IE=EDGE"); ?>
<!DOCTYPE html>
. . .
But the HTML validator complains -
Error: X-UA-Compatible HTTP header must have the value IE=edge, was IE=9; IE=8; IE=7; IE=EDGE.
What is the correct syntax if I do not want to ignore IE 7, 8, 9 users?
Use content="ie=edge", which will force IE8 and later to use the highest mode they can support, e.g. IE9 standards mode in IE9, IE8 standards mode in IE8, and so on.
For IE7 (and the others, really), you should also use <!DOCTYPE html> for the document type declaration. This tells IE7 (and IE6) to also use the best support they have available.
Hope this helps...
-- Lance