Line 30, Column 57: there is no attribute "DATA-FLEXMENU"
… href="about.php" class="mainlink" data-flexmenu="flexmenu1">About Us
Line 30, Column 57: there is no attribute "DATA-FLEXMENU"
… href="about.php" class="mainlink" data-flexmenu="flexmenu1">About Us
The problem is that the current doctype you're using doesn't support the data-flexmenu attribute.
You should either take the data-flexmenu attribute out of your href, switch to an HTML5 doctype (it supports the data attribute), or just live with the error.