Centering text within a responsive image - html

Question

I am trying to make a showcase section for a web page. It consists of a div with a (responsive) background image and a header that would be centered horizontally and vertically over this image. I've managed to get the image in and have it be responsive, and I've got the header centered, but my problem arises when the window size becomes smaller.
I'm using position: absolute, the top property, and transform to have it be centered, but the top property only works when height is specified in the parent container. However, when the window shrinks to the point where the image begins to shrink to below its original height, the text does not stay vertically centered, only horizontally (since I'm going off of the original height for top (800px)).
I can't just change the height with a media query since the image size is changing constantly and I can't not use height because then the top property would not work at all, so I'm a bit confused with how to get around this.
Here are the relevant sections of my code:
HTML:
<section class="showcase">
<div class="showcase-container">
<h1 class="centered"><span class="highlight">BR</span> Architects</h1>
</div>
</section>
CSS:
.container {
width: 80%;
margin: 0 auto;
padding: 0;
position: relative;
height: auto;
}
.showcase-container {
width: 80%;
margin: 0 auto;
padding: 0;
position: relative;
height: 800px;
}
.centered {
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
}

Answer 1

I might just guess because I don't know how does this really look, but I assumed few things and in a result instead of background image I would just use normal image, make it blocky and display div over it, you will have height preserved in any size, take a look:
.showcase-container {
width: 80%;
margin: 0 auto;
padding: 0;
position: relative;
}
.showcase-container img {
display: block;
width: 100%;
height: auto;
margin: 0 auto;
max-width: 1200px;
}
.centered {
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
background: #fff;
border-radius: 5px;
padding: 10px 20px;
}
<section class="showcase">
<div class="showcase-container">
<img src="https://source.unsplash.com/random/1200x700" alt="">
<h1 class="centered"><span class="highlight">BR</span> Architects</h1>
</div>
</section>

Answer 2

See MDN's <figcaption> documentation.
<figure>
<img src="/media/examples/hamster.jpg" alt="a cute hamster" />
<figcaption>Hamster by Ricky Kharawala on Unsplash</figcaption>
</figure>

Answer 3

If I'm understanding this right, you're saying you don't need to worry about the image always maintaining an 800px height, you just want the h1 to remain centered. In that case, it's really simple.
Just add your image as a background, setting the background-size to cover, then make sure the container is never larger than the window by setting its height to 100vh, but never taller than 800px by setting its max-height.
.showcase-container {
/* your styles here */
background-image: url('yourimage.jpg');
background-size: cover;
height: 100vh;
max-height: 800px;
}
OR if you need it to be vertically centered in the window independently of the container, you can always change top: 50%; to top: 50vh; and position relative to the body.