Демонстрация border

https://webref.ru/layout/learn-html-css/box-model

by webref

HTML

<code class="border-solid">2px <br> solid</code>

<code class="border-double">6px <br> double</code>

<code class="border-dashed">8px <br> dashed</code>

<code class="border-dotted">4px <br> dotted</code>

CSS

code {
  background: #eaeaed;
  color: #666;
  font: 14px/24px "Source Code Pro", Inconsolata, "Lucida Console", Terminal, "Courier New", Courier;
  display: inline-block;
  height: 70px;
  margin: 0 14px;
  padding-top: 20px;
  text-align: center;
  width: 90px;
}
.border-solid {
  border: 2px solid #9799a7;
}
.border-double {
  border: 6px double #9799a7;
}
.border-dashed {
  border: 8px dashed #9799a7;
}
.border-dotted {
  border: 4px dotted #9799a7;
}