CSS

Replace<div class="card">with the main tag, <p class="card-text"> with <h1> and <div class="attribution"> with the footer tag to fix the accessibility issues. click here for more on web-accessibility and semantic html

To center .card on the page, add min-height:100vh; display: flex; align-items: center: justify-content: center; or min-height:100vh; display: grid place-items: center to the body.

To center .card on the page using flexbox:
body{
min-height: 100vh;
display: flex;
align-items: center;
justify-content: center;
}

To center .card on the page using grid:
body{
min-height: 100vh;
display: grid;
place-items: center;
}

increase the width of .card and its padding value. Give .card-body text-align-center, a margin value for all the sides and font-size of 15px, this will be the font-size of p and h1.

Use relative units like rem or em as unit for the padding, margin, width values and preferably rem for the font-size values, instead of using px which is an absolute unit. For more on CSS units Click here

Hope am helpful.

Well done for completing this challenge. HAPPY CODING