How do I give a CSS octagon shape a full border?

Question

I am trying to take this css Octagon and give it a solid red border. However, when i add the border it only works on portion of the shape. Does anyone have a solution for this?

Here is a JS FIDDLE

HTML

<div class='octagonWrap'>
      <div class='octagon'></div>
  </div>

CSS

.octagonWrap{
        width:200px;
        height:200px;
        float: left;
        position: relative;
        }
    .octagon{
        position: absolute;
        top: 0; right: 0; bottom: 0; left: 0;
        overflow: hidden;
        }
    .octagon:before {
        position: absolute;
        top: 0; right: 0; bottom: 0; left: 0;
        transform: rotate(45deg);
        background: #777;
        content: '';
        border: 3px solid red;
        }

Answer 1

You can modify your own code to work with this. Right now, you have rules for .octagon that should be for .octagon-wrapper, and rules for .octagon::before that should be for .octagon. Just shift the CSS rules and have them apply to their parents, while changing the border property of .octagon::before to inherit from .octagon.

.octagonWrap {
    width:200px;
    height:200px;
    float: left;
    position: relative;
    overflow: hidden;
}
.octagon {
    position: absolute;
    top: 0; right: 0; bottom: 0; left: 0;
    overflow: hidden;
    transform: rotate(45deg);
    background: #777;
    border: 3px solid red;
}
.octagon:before {
    position: absolute;
    /* There needs to be a negative value here to cancel
     * out the width of the border. It's currently -3px,
     * but if the border were 5px, then it'd be -5px.
     */
    top: -3px; right: -3px; bottom: -3px; left: -3px;
    transform: rotate(45deg);
    content: '';
    border: inherit;
}

<div class='octagonWrap'>
    <div class='octagon'></div>
</div>

Answer 2

SVG solution

I do not know if using svg is an option,
If it is here is how simple it is done.

<svg viewBox="0 0 75 75" width="200px">
  <path d="m5,22 18,-18 28,0 18,18 0,28 -18,18, -28,0 -18,-18z" stroke="red" stroke-width="2" fill="black" />
</svg>