W3C validation

Hello
I spent a lot of time to validate my blog using W3c tools , I got different errors and warning and all of them about the core of WP it-self , on of these error
there is no attribute “role”
<form role="search" method="get" id="searchform"
I try to make a Validation Checklist as WP Community stat.
Validation Checklist
Is there any method to validate my blog using the correct syntax for both W3C standardizations with no-arbitrary change of wp files or functions.
another thing I view source of WP site i find they use transitional DTD type , while the WP header.php contain Strict DTD type. what is the purpose of WP Developers for using different dtd type between their site and their product.

Best Regard.