As a general rule, we should give the user as much control as possible, and we should never disable or block their settings from working. For this reason, it's very important to use a relative unit like rem for typography.

Then we should use rem everywhere ?

Do I actually want everything to scale with font size?
When we use rem values for horizontal padding, though we amplify the effect "larger the text is, the fewer characters can fit on each line".

So the final question is:

“Should this value scale up as the user increases their browser's default font size?”

I've come to realize, however, that we usually do want to use rems for media queries.

We're so used to thinking of media queries in terms of mobile/tablet/desktop, but I think it's more helpful to think in terms of available space. [Why in the post]

Other example is the space between paragraphs: it has a "functional" purpose so that we can quickly tell where one paragraph ends and the next one begins. For this reason, it does make sense to scale these margins with the user's chosen root font size.

Example of use of em instead of rem

• rem

h1 {
  font-size: 3rem
  margin-top: 6rem;
  margin-bottom: 1.5rem;
}
h2 {
  font-size: 2rem
  margin-top: 4rem;
  margin-bottom: 1rem;
}
h3 {
  font-size: 1.5rem;
  margin-top: 3rem;
  margin-bottom: 0.75rem;
}

• em

h1 {
  font-size: 3rem;
}
h2 {
  font-size: 2rem;
}
h3 {
  font-size: 1.5rem;
}
h1, h2, h3 {
  margin-top: 2em;
  margin-bottom: 0.5em;
}

Another example is the width of a button: we can think about it alone yet.

Finally, it is better to forge an intuition as opposed to a set of rules about using rem/px as there are always edge cases.