Modify "head-placeholder" in html.html.twig that's the first time I approach to Drupal or a CMS more generally but I've been working since years as FE dev.

I'm creating a theme based on a template created from scratch for the Italian Public Administration(PA) in such way to define a standard for the PA websites.

This is the template project

So I have to fully rely over the CSS and JS files provided by the theme, and can't use a base theme.

I'm struggling to understand how to change the content of the head-placehoder and the title one in the html.html.twig file

  &lt;head&gt;
    &lt;head-placeholder token="{{ placeholder_token }}"&gt;
    &lt;title&gt;{{ head_title|safe_join(' | ') }}&lt;/title&gt;
    &lt;css-placeholder token="{{ placeholder_token }}"&gt;
    &lt;js-placeholder token="{{ placeholder_token }}"&gt;
  &lt;/head&gt;
I suggest you look at https://www.drupal.org/docs/8/theming-drupal-8/adding-stylesheets-css-and-javascript-js-to-a-drupal-8-theme, which provides pretty good documentation on how to add CSS in a theme.

If you don't extend from a theme then you technically extend from a special theme called "stable". You should get almost no default CSS then, at least from core modules. Unless that really results in a problem, in which case there are some options to override it (documented above), I wouldn't worry about that.