Add centered text to the middle of a <hr/>-like line

How about:

<div style="width: 100%; height: 20px; border-bottom: 1px solid black; text-align: center">
  <span style="font-size: 40px; background-color: #F3F5F6; padding: 0 10px;">
    Section Title <!--Padding is optional-->
  </span>
</div>

Check out this JSFiddle.

You can use vw or % to make it responsive.

The way to solve this without knowing the width and the background color is the following:

Structure

<div class="strike">
   <span>Kringle</span>
</div>

CSS

.strike {
    display: block;
    text-align: center;
    overflow: hidden;
    white-space: nowrap; 
}

.strike > span {
    position: relative;
    display: inline-block;
}

.strike > span:before,
.strike > span:after {
    content: "";
    position: absolute;
    top: 50%;
    width: 9999px;
    height: 1px;
    background: red;
}

.strike > span:before {
    right: 100%;
    margin-right: 15px;
}

.strike > span:after {
    left: 100%;
    margin-left: 15px;
}

Example: http://jsfiddle.net/z8Hnz/

Double line

To create a double line, use one of the following options:

1) Fixed space between lines

.strike > span:before,
.strike > span:after {
    content: "";
    position: absolute;
    top: 50%;
    width: 9999px;
    border-top: 4px double red;

Example: http://jsfiddle.net/z8Hnz/103/

2) Custom space between lines

.strike > span:before,
.strike > span:after {
    content: "";
    position: absolute;
    top: 50%;
    width: 9999px;
    height: 5px; /* space between lines */
    margin-top: -2px; /* adjust vertical align */
    border-top: 1px solid red;
    border-bottom: 1px solid red;
}

Example: http://jsfiddle.net/z8Hnz/105/

SASS (SCSS) version

Based on this solution, I added SCSS "with color property" if it could help someone...

//mixins.scss
@mixin bg-strike($color) {

    display: block;
    text-align: center;
    overflow: hidden;
    white-space: nowrap; 

    > span {

        position: relative;
        display: inline-block;

        &:before,
        &:after {
            content: "";
            position: absolute;
            top: 50%;
            width: 9999px;
            height: 1px;
            background: $color;
        }

        &:before {
            right: 100%;
            margin-right: 15px;
        }

        &:after {
            left: 100%;
            margin-left: 15px;
        }
    }
}

example of use :

//content.scss
h2 {
    @include bg-strike($col1);
    color: $col1;
}

Flexbox is the solution:

.separator {
  display: flex;
  align-items: center;
  text-align: center;
}

.separator::before,
.separator::after {
  content: '';
  flex: 1;
  border-bottom: 1px solid #000;
}

.separator:not(:empty)::before {
  margin-right: .25em;
}

.separator:not(:empty)::after {
  margin-left: .25em;
}

<div class="separator">Next section</div>

Nowadays every browser supports it, and you can ensure compatibility with decade-old browsers by adding respective vendor prefixes if needed. It would degrade gracefully anyways.