Center a `.img-responsive`, in a `.container-fluid` and `row`, in bootstrap? - css

Question

I don't expect the answers here to add much to the 21st century's understanding of computer science, but I am grateful for the help of the community.
I have an image I'm trying to center in bootstrap. It's in a .container-fluid, a .row, and a .col-xs-6 with .offset-xs-3. The <img...> itself is .img-responsive. You can see the project on this codepen: http://codepen.io/NotAnAmbiTurner/pen/GrGmaq?editors=1000.
The image is not centering - I should be able to get an image that scales with the viewport width (a la SVG) and is in the middle of the page. I can't seem to manage that.
I'm wondering if it has something to do with the fact that I'm taking a "raw" image from dropbox as my src, that the image is a .jpg, or some combination? Then again, I'm a noob at this, so it's equally possible that there's just a typo somewhere I can't seem to find. I have been over the bootstrap docs, and various SO questions to no avail. The only solution I have found is removing the <img...> from the container and row, and assigning ... class="centered-block" .... Unless I'm fundamentally misapprehending something, however, (1) I shouldn't have to do that, and (2) rather than bashing through I'd like to understand what my conceptual error is in any case, for my own learning.
PS - totally unrelated, but my <a ...></a> tags don't seem to be clickable. If anyone has any ideas there, that would be great too.
It turns out the basic issue is that CodePen relies on Bootstrap 4, not (as I had assumed) Bootstrap 3.

Answer 1

You're using classes from Bootstrap 3 but loading the stylesheet from Bootstrap 4; they aren't compatible across the board. img-responsive no longer exists as well as the col-xs-* classes. See Images and Grid Options.
Note: Your links won't work because they aren't valid HTML and you're also closing your h2 with an h3, validate your markup.
Working Example:
#media (max-width: 480px) {
h1 {
font-size: 5vmax;
}
h2 {
font-size: 3.5vmax;
}
h3 {
font-size: 2.5vmax;
}
li {
font-size: 2.5vmax;
}
}
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css" rel="stylesheet" />
<div class="jumbotron">
<h1 class="text-primary">Dr. Clair Cameron Patterson</h1>
<h2 class="text-secondary">The man who first learned the age of the Earth, and helped save a civilization from the dangers of its use of lead.</h2>
<div class="container-fluid">
<div class="row">
<div class="col">
<img src="https://www.dropbox.com/s/91fw6aazyxzb178/image-for-codecamp-tribute-page.jpg?raw=1" alt="Cartoon Image of Clair Cameron Patterson at Congressional Hearing" class="img-fluid mx-auto d-block" style="padding:1.5vmin">
</div>
</div>
</div>
<h3>Academics</h3>
<ul>
<li>Undergraduate, Grinnell College</li>
<li>PhD, University of Chicago</li>
<li>Researcher, California Institute of Technology</li>
</ul>
<h3>Brief Timeline</h3>
<ul>
<li><strong>1956</strong>: found the earth to be 4.55 Billion years old using lead-lead dating and
a fragment of a metorite that fell in Arizona. This calculation has been largely undisturbed since.</li>
<li><strong>1965</strong>: began campaign against lead in various products, especially gasoline, which at that time was emitting lead into the atmosphere.</li>
<li><strong>1978</strong>: wrote a 78-page minority opinion as part of a National Research Council (NRC) panel stating that controls on lead in various products had to start immediately, including gasoline, food containers, paint, glazes, and water distribution
systems.
</li>
<li>by the late <strong>1990s</strong>: levels of lead in Americans' blood has dropped by up to 80%. Though connections with Dr. Patterson's work are not clear, as the leading researcher in the area, and constant campaigner against the use of lead, it
is likely he was a major antecedant of this change.</li>
</ul>
</div>