What is placeholder_token in html.html.twig for?
The reason why placeholder_token is used in html.html.twig instead of using just a normal variable is that in Drupal 8 we have different render strategies than just the simple rendering where everything gets rendered in the template and we are able to collect the assets.

In the Drupal cores default Renderer we have placeholders (placeholders might attach some additional assets). If we would print the assets in a normal variable it would happen before all the assets have been collected. So what Drupal 8 does is it first renders everything with the normal rendering in the template, then it replaces the placeholders and collects assets from then, and after all that has been done we finally replace the asset placeholders to add the assets to the page.