Use before & after Pseudo-element to make a line

Question

I'm using Pseudo-element :before and :after to draw a line before and after a title. It's working with an image:

.mydiv::before {
content: url(img/line.png);}
.mydiv::after {
content: url(img/line.png);}

Here is the result :

But, I would like the line to expand and fill in the whole div before and after the title, like this :

Is there a way to specify a percentage for the image for it to stretch? I try this, but it's not working :

.mydiv img {
  width: 100%;
  height: auto;
}

Answer 1

You don't need both :before and :after, either of the 2 will be enough and as you've been told, you don't need an image. See the approach below.

#header {
    width: 100%;
    height: 50px;
    margin: 50px 0;
    text-align: center;
    font-size: 28px;
    position: relative;
    background-color: #57585C;
}

#header:after {
    content: '';
    width: 100%;
    border-bottom: solid 1px #fff;
    position: absolute;
    left: 0;
    top: 50%;
    z-index: 1;
}

h3 {
    background-color: #57585C; /* Same as the parents Background */
    width: auto;
    display: inline-block;
    z-index: 3;
    padding: 0 20px 0 20px;
    color: white;
    position: relative;
    font-family: calibri;
    font-weight: lighter;
    margin: 0;
}

<div id="header">
   <h3>Last Projects</h3>
</div>

Answer 2

In case you need <h3> title to have transparent background - you can use both :before and :after and display: flex

More about flex-grow you can read here https://developer.mozilla.org/en-US/docs/Web/CSS/flex.

body {
  background: linear-gradient(0.25turn, #3f87a6, #000000, #f69d3c); /* example of custom background */
}

#header {
  display: flex;
  flex-wrap: nowrap;
  justify-content: flex-start;
  align-items: center; /* making vertical centerign of all children */
}

#header::before, #header::after {
  content: '';
  flex: 1 1 auto; /* the first digint is 'flex-grow: 1', helps elemet to occupy all free space */ 
  border-bottom: solid 1px #fff;
}

h3 {
  flex: 0 1 auto; /* the first digint is flex-grow: 0 */ 
  padding: 0 15px 0 15px;
  color: #fff;
}

<div id="header">
  <h3>Last Projects</h3>
</div>