For anyone else covering the same dilemma and doesn't care about transitions:

https://codepen.io/gschier/pen/HCoqh

Or, reduced further:

CSS

    .lightbox {
	display: none;
	position: fixed;
	z-index: 999;
	width: 100%;
	height: 100%;
	text-align: center;
	top: 0;
	left: 0;
	background: rgba(0,0,0,0.8);
    }

    .lightbox img {
	max-width: 90%;
	max-height: 80%;
	margin-top: 2%;
    }

    .lightbox:target {
	outline: none;
	display: block;
    }

    <a href="#img1">
      <img src="image.png" class="thumbnail">
    </a>

    <a href="#_" class="lightbox" id="img1">
      <img src="image.png">
    </a>

(See CodePen link for code credit. Pretty clever little item, if you don't have to parse the pure markup. Even then it's not overly intrusive.)

Source: I'm currently building out my own Javascript-light blog/cv site, even though I generally like working with React. It's just not needed for everything. I'll likely extend on this at least a bit for some nicer effect with some current Javascript— barring older browsers from the lightbox effect but still able to read the thing in lynx and if need-be.