This include splits the content of {{ content }} into two parts if Jekyll finds an excerpt separator. This allows extra styling of the first part to enlarge the the fontsize of the teaser for example.

automatic_post_excerpt_for_post_pages.liquid

{% comment %}
#
#   Explanation:
#
#   This include splits the content of {{ content }} into
#   two parts if Jekyll finds an excerpt separator. This allows
#   extra styling of the first part to enlarge the the fontsize
#   of the teaser for example.
#
#   The excerpt separator can be defined directly in your
#   _config.yml or via front matter. If no separator is defined
#   Jekyll will fall back to the default separator <!--more-->.
#   If no separator is set, Jekyll just spits out the content
#   without splitting.
#
{% endcomment %}
{% capture page_content %}{{ content }}{% endcapture %}
{% if site.excerpt_separator %}{% assign separator = site.excerpt_separator %}{% else %}{% assign separator = '<!--more-->' %}{% endif %}

{% if page_content contains separator %}
  {% assign entry = page_content | split: separator %}

  {{ entry | first | remove: '<p>' | prepend: '<p class="lead">' }}
  {{ entry | last }}

{% else %}
  {{ page_content }}
{% endif %}