Building a QR Code design

I am absolutely new to HTML and CSS so I find it quite overwhelming at first. Though I managed to complete it in 3 hours, I'm not quite sure if my solution is good. As a disclaimer, I think my solution can only be opened on a desktop browser (I'm not entirely sure).

There's this particular part that I don't really understand:

.attribution {
    font-size: 11px;
    text-align: center;
    position: absolute;
    bottom: 0;
    width: 99%;
    padding: 20px 0;
}

It was supposed to put the text at the center bottom of the body but, if I put width: 100% It's not quite at the center.

The HTML part is here:

<div class="attribution">
      Challenge by
      <a href="https://www.frontendmentor.io?ref=challenge" target="_blank"
        >Frontend Mentor</a
      >. Coded by <a href="#">Nurul Mohamad</a>.
</div>

Also, any advice on how to learn HTML and CSS effectively is welcome!