Serving CSS, images and JavaScript in Drupal Does the whole Drupal stack get fired up (bootstrapped) each time one of these static content types gets requested?
Nope, it does not.
But if you have let's say in one of your stylesheet files a reference to a missing image, or your code is trying to load a missing js file then because of the Rewrite Rules in .htaccess this requests are redirected to index.php and that causes the bootstrapping of the whole system to render the 404 page.