Card Design with Bootstrap.

What units to use in what situation really depends on what you want to do with a given design. However, pixels should be avoided in most situations except for cases where you don't want to resize something (for instance, border-sizes). That being said, I found this video to be very helpful:

Are you using the right CSS units?

For your second question I would make the body element into a flexbox and extend it the full height of the window like so:

body{
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
}

You'll also need to remove the margin-top (or set it to 0) on the .card class.

Hi May, welcome to the Frontend Mentor community, and great job in completing your very first challenge! 😊

I think @JordanKisiel gave you some excellent advice, especially the link to Kevin Powell's explanation on which units to do, so do be sure to have a look at that when you can. There are times when you may want to use pixels, but for the majority of the time I think relative units are the best choice.

As for feedback on your work, I really think you did many things well here for your first challenge, and the only two suggestions I have are (1) change the first div to main to take care of the landmark issue in your report, and (2) add a margin: 1rem to your .card so that there's some spacing around the card when the browser size gets smaller. I tried adding the margin via the inspector, and then I see that you got some important tags on the margin property. I would say try not to use important tags if you can help it, and in fact, whenever possible, try to avoid using them. If you find yourself needing to use them, then actually what you need to do is figure out which property or rule is cause the declaration to not work as expected.

Hope some of this can help you out, and keep going, you're doing great!