Как это сверстать?

Question

[obl]

как сверстать нижнюю полоску ?

Answer 1

[boga-net]

https://jsfiddle.net/ouodkeLn/93/

или так

https://jsfiddle.net/ouodkeLn/96/

или так

https://jsfiddle.net/ouodkeLn/99/

Answer 2

[Dymok]

.button {
  ...
  position: relative;
}

.button:after {
  content: '';
  width: 100vw;
  height: 1px;
  position: absolute;
  bottom: 0;
  left: 0;
  background: red;
}

Answer 3

[YuraArt]

<div class="button-wrap">
    <button>Lorem ipsum.</button>
</div>

.button-wrap{
    border-bottom: 1px solid red;
}
.button-wrap button{
    ...
    margin-bottom: -1px;
}

Answer 4

[DmitriySwan]

https://jsfiddle.net/6Lgm08kn/