How should I include  or  inside ? I'm trying to use a Google font, which requires adding either a &lt;link&gt; or &lt;style&gt; section inside the &lt;head&gt; per the below instructions. I am using a subtheme and can modify the  section directly in html.html.twig, if that's the right way to do it.

However, I noticed &lt;head-placeholder&gt;, &lt;css-placeholder&gt;, and &lt;jss-placeholder&gt; inside &lt;head&gt; and I am wondering if those are intended to be used to insert code from somewhere else. 

What's the best practice for adding to &lt;head&gt;?
First, define a library in your theme that links to the font.

[THEME].libraries.yml:

fonts:
  css:
    theme:
      # Do not include http: or https.
      # Should start with a double slash.
      //fonts.googleapis.com/css?family=Source+Sans+Pro: { type: external }


This library is loaded by adding it to the theme's [THEME].info.yml file:

libraries:
  - [THEME]/fonts