site slogan in h6 following h2 site name not matching WCAG 2.0 requirements
Hooks won't help you with this - the markup for the slogan is in the region--branding.tpl.php template file:

&lt;h6 class="site-slogan&lt;?php print $class; ?&gt;"&gt;&lt;?php print $site_slogan; ?&gt;&lt;/h6&gt;


To fix, you can just create your own region--branding.tpl.php file in your sub-theme, and alter the markup to your needs.