font-variant-ligatures「合字の制御指定」

<style type="text/css">
.common div{
  font-family   : "メイリオ", sans-serif;
  width         : 290px;
  border        : 2px solid #6495ed;
  background    : #e7e7ff;
  padding       : 10px;
}
div.cssTitle {
  margin-top    : 10px;
  background    : #6495ed;
  color         : #fff;
  font-weight   : bold;
}
.css-liga-clig{font-feature-settings : "liga","clig";}
.css-dlig{font-feature-settings : "dlig";}

.css-ligatures-normal{font-variant-ligatures : normal;}
.css-ligatures-common{font-variant-ligatures : common-ligatures;}
.css-ligatures-discretionary{font-variant-ligatures : discretionary-ligatures;}

.css-variant-common{font-variant: common-ligatures;}
.css-variant-discretionary{font-variant: discretionary-ligatures;}

<!-- html -->
<div class="common">
  <div class="cssTitle">一般的な合字<br>
  font-variant-ligatures: common-ligatures</div>
  <div>
  <span class="css-ligatures-normal">
  ff や fi が1つの字形になる</span><br>
  <span class="css-liga-clig">
  ff や fi が1つの字形になる</span><br>
  <span class="css-ligatures-common">
  ff や fi が1つの字形になる</span><br>
  <span class="css-variant-common">
  ff や fi が1つの字形になる</span>
  </div>
  <div class="cssTitle">任意の合字<br>
  font-variant-ligatures: discretionary-ligatures</div>
  <div>
  <span class="css-ligatures-normal">
  ct や sp が1つの字形になる</span><br>
  <span class="css-liga-dlig">
  ct や sp が1つの字形になる</span><br>
  <span class="css-ligatures-discretionary">
  ct や sp が1つの字形になる</span><br>
  <span class="css-variant-discretionary">
  ct や sp が1つの字形になる</span>
  </div>
</div>