If you view the source you will see a w right before the DOCTYPE
w
<!DOCTYPE html>
<!--[if IE 7]> <html class="ie ie7" lang="en-US" prefix="og: http://ogp.me/ns#"> <![endif]-->
<!--[if IE 8]> <html class="ie ie8" lang="en-US" prefix="og: http://ogp.me/ns#"> <![endif]-->
<!--[if IE 9]> <html class="ie ie9" lang="en-US" prefix="og: http://ogp.me/ns#"> <![endif]-->
<!--[if (gt IE 9)|!(IE)]><!-->
<html lang="en-US" prefix="og: http://ogp.me/ns#">
Check your header.php file as this is likely where the w is.