How to access $vars['some'] variables in html.tpl.php template
The general rule for any THEMEHOOK.tpl.php file (where THEMEHOOK is something like 'html', 'page', 'node' etc.) is:


The default variables are set in template_preprocess_THEMEHOOK() (eg.
template_preprocess_html)
Any module can add variables using MODULENAME_preprocess_THEMEHOOK() (eg. toolbar_preprocess_html)
The active theme can add variables using THEMENAME_preprocess_THEMEHOOK() (eg. bartik_preprocess_html)


The $variables array (sometimes called $vars) is passed on from function to function in the above order, so each function can expand on or overwrite the variables added before.