Line before and after text not responsive [duplicate]

Answer 1

You can consider the pseudo element relative to h2 instead and rely on overflow to hide the non needed parts:

.section-header {
  position: relative;
  overflow:hidden; /*mandatory*/
}

.section-header h2 {
  border: 2px solid black;
  padding: 0.3em 0.8em;
  display: inline-block;
  position:relative;
}

.section-header h2::before,
.section-header h2::after {
  content: " ";
  width: 100vw; /*big value here*/
  height: 1px;
  position: absolute;
  top: 0.6em;
  background:black;
}

.section-header h2::after {
  left: calc(100% + 40px); /*40px offset from the title*/
}
.section-header h2::before {
  right: calc(100% + 40px);
}

.text-center {
    text-align: center;
}

<div class="section-header text-center">
  <h2>Testing</h2>
</div>

Another idea without transparency is to consider background and box-shadow like below:

.section-header {
  position: relative;
  background:linear-gradient(black,black) center/100% 1px no-repeat;
}

.section-header h2 {
  border: 2px solid black;
  padding: 0.3em 0.8em;
  display: inline-block;
  position:relative;
  background:#fff;
  box-shadow: 
    40px 0 0 #fff,
    -40px 0 0 #fff;
}
.text-center {
    text-align: center;
}

<div class="section-header text-center">
  <h2>Testing</h2>
</div>

Answer 2

A simple option is to use a flexbox for section-header - then you can:

• set the space between the h2 and the lines using a margin set to the h2
• set the width of the pseudo elements as 100% - being a flex item, it will adapt to the space available

See demo below:

.section-header {
  position: relative;
  display: flex; /* sets a flex container */
  align-items: center; /* aligns vertically */
}

.section-header h2 {
  border: 2px solid rgba(0, 0, 0, 0.1);
  padding: 0.3em 0.8em;
  display: inline-block;
  margin: 0 1em; /* space between h2 and line if needed */
}

.section-header::before,
.section-header::after {
  border-top: 1px solid black;
  display: block;
  height: 1px;
  content: " ";
  width: 100%; /* full-width ;)*/
  top: 1.2em;
  opacity: 0.1;
}

.text-center {
  text-align: center !important;
}

<div class="section-header text-center">
  <h2>Testing</h2>
</div>