<p><span class="mark1">マークしたい部分</span>に黄色い線をいれます</p>
<p><span class="mark2">マークしたい部分</span>に黄色い線をいれます</p>
<p><span class="mark3">マークしたい部分</span>に黄色い線をいれます</p>
<p><span class="mark4">マークしたい部分</span>に黄色い線をいれます</p>
<p><span class="mark5">マークしたい部分</span>に黄色い線をいれます</p>
.mark1{ background: linear-gradient(transparent 0%, #f6ff5f 0%);}

.mark2{ background: linear-gradient(transparent 20%, #f6ff5f 20%);}
  
.mark3{ background: linear-gradient(transparent 40%, #f6ff5f 40%);}

.mark4{ background: linear-gradient(transparent 60%, #f6ff5f 60%);}

.mark5{ background: linear-gradient(transparent 80%, #f6ff5f 80%);}

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