css/modules/primer-markdown/lib/markdown-body.scss

// All of our block level items should have the same margin
// stylelint-disable selector-no-type
$margin: 16px;

// This is styling for generic markdownized text. Anything you put in a
// container with .markdown-body on it should render generally well. It also
// includes some GitHub Flavored Markdown specific styling (like @mentions)
.markdown-body {
  font-family: $body-font;
  font-size: 16px;
  line-height: $body-line-height;
  word-wrap: break-word;

  // Clearfix on the markdown body
  &::before {
    display: table;
    content: "";
  }

  &::after {
    display: table;
    clear: both;
    content: "";
  }

  > *:first-child {
    margin-top: 0 !important;
  }

  > *:last-child {
    margin-bottom: 0 !important;
  }

  // Anchors like <a name="examples">. These sometimes end up wrapped around
  // text when users mistakenly forget to close the tag or use self-closing tag
  // syntax. We don't want them to appear like links.
  // FIXME: a:not(:link):not(:visited) would be a little clearer here (and
  // possibly faster to match), but it breaks styling of <a href> elements due
  // to https://bugs.webkit.org/show_bug.cgi?id=142737.
  a:not([href]) {
    color: inherit;
    text-decoration: none;
  }

  // Link Colors
  .absent {
    color: $red-600;
  }

  .anchor {
    float: left;
    padding-right: 4px;
    margin-left: -20px;
    line-height: 1;

    &:focus {
      outline: none;
    }
  }

  p,
  blockquote,
  ul,
  ol,
  dl,
  table,
  pre {
    margin-top: 0;
    margin-bottom: $margin;
  }

  hr {
    height: 0.25em;
    padding: 0;
    margin: ($margin * 1.5) 0;
    background-color: $gray-200;
    border: 0;
  }

  blockquote {
    padding: 0 1em;
    color: $gray-500;
    border-left: 0.25em solid lighten($gray-300, 5%);

    > :first-child {
      margin-top: 0;
    }

    > :last-child {
      margin-bottom: 0;
    }
  }

  kbd {
    display: inline-block;
    padding: 3px 5px;
    font-size: 11px;
    line-height: 10px;
    color: $gray-700;
    vertical-align: middle;
    background-color: $gray-000;
    border: solid 1px darken($gray-300, 4%);
    border-bottom-color: $gray-400;
    border-radius: 3px;
    box-shadow: inset 0 -1px 0 $gray-400;
  }
}