Browsers nowadays perfectly zoom in and out when using cmd-+ and cmd--. Because of this, developers sometimes think that using relative units is actually not necessary. But as zooming that way only affects the current website, browsers also have a setting to change the default font size of the browser. Unfortunately, this has no effect when using absolute units. That is why using relative units is important.

As in the beginning of my developer career it was still completely normal to use px for everything and getting Photoshop layouts with pixel measurements, I still think in pixels when developing. Of course, there are tools that help you with that: you can e.g. use px in your code base which becomes converted to rem when using a post- or pre-processor.

But as it is better to not rely on tooling, I prefer to use one of my all-time favourite css tricks:

html {
  font-size: 62.5%;
}

62.5% equals 10px, as the default font size of a browser is 16px (16 * 0,625 = 10).

Now you can use rem and your values are the same as your pixel values, the decimal separator just moved one digit to the left:

body {
  font-size: 1.6rem; /* 16px */
}

.wrapper {
  max-width: 72rem; /* 720px */
}

Don’t forget to explicitely set font-size on the body now, as 10px is too small for a default value.

Note: I sometimes see developers using html: { font-size: 10px; } to achieve the same, but this does not fix the zooming problem.