3-column preview card component

Replace <div class="big_box"> with the main tag and the html must have <h1> to fix the accessibility issues. click here for more on web-accessibility and semantic html

to center .big box on the page using flexbox or grid instead position: relative, 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 .big-box on the page using flexbox:
body{
min-height: 100vh;
display: flex;
align-items: center;
justify-content: center;
}

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

There is no need to give .big box a height value rather give the boxes the same padding value for all the sides.

you forgot to add a media query for the mobile desidn, for more on media queries, click here

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