What does the fixed font rule mean?

Question

css

In one old article about SEO optimization, I subtracted the rule:

You can not design with a fixed font size of the main text

Since I don’t really rummage in the topic of design, layout and SEO, I would like to ask the guru what does it mean? You have to write like:

body {
font-size: 120%;
}

but not

body {
font-size: 16 px;
}

?
And one more thing: how relevant is this rule at the present time?
Thanks in advance to all who answer.

Answer 1

The main idea is that the user in the browser can override the default font size. For example, he does not see well and made bigger letters for himself, or he has a 4K monitor.
If you set the base font size in pixels, this setting does not work for the user and the letters will be too small. On the other hand, I have not met users who change exactly the font size, and do not use CTRL+mouse wheel scaling.
Learn about CSS units rem and em.