I simply finished create my very own wordpress theme. However when I checked the html validation through validator.w3.com, it got:

502 Proxy Error The proxy server received an invalid response from an upstream server. The proxy server couldn't handle the request GET /check.

Reason: Error reading through from remote server. Apache/2 Server at validator.w3.org Port 80

Exactly what does it mean? So far as I understand, I do not experiment using any proxy server.

----- EDIT: -----

Appears like the issue is in functions.php:

if(isset($_GET['page']) && $_GET['page']=='admin-functions.php'){
**codes here***
}

Basically take away the if(isset($_GET['page']) &lifier&lifier $_GET['page']=='admin-functions.php'), it returns no 502 proxy error. How must i fix this code to cope with the w3 validator?

Thank you!

----- EDIT: -----

well i guess, apparently it isn't the issue. Even when I take away the if(isset($_GET['page']) &lifier&lifier $_GET['page']=='admin-functions.php'), often it returns 502 error, but sometimes not. I truly do not get the concept.

-----FOUND The Answer!-----

call me stupid!

I discovered the issue: I personally use these DOCTYPEs:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

i quickly removed these and alter to :

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

Works fine now