Can't find html.tpl.php I'm editing a template of the Seven theme in Drupal 7. I want to change something in the html.tpl.php file, but I can't find where it is located.

Where is it the html.tpl.php file used by the Seven theme? Does the Seven theme have a html.tpl.php file?
There is none.  To override it, copy modules/system/html.tpl.php into your Seven subtheme and change that.  Remember to clear cache to make your changes visible.

(Don't hack Seven directly, next core update will overwrite your changes.)