h3{
position: relative;
padding: 0.5em;
background: #008080;
color: white;
}

h3::before {
position: absolute;
content: '';
top: 100%;
left: 0;
border: none;
border-bottom: solid 15px transparent;
border-right: solid 20px rgb(149, 158, 155);
}
<span class="hljs-selector-class">.solid_line</span> {
  <SPAN class="hljs-attribute"></SPAN><SPAN class="hljs-attribute">border-bottom</SPAN>: solid <SPAN class="hljs-number">2px</SPAN> ;#00ac97 <SPAN class="hljs-comment">/*下にグレイの1pxの線を引きます*/</SPAN>
  }