What's the Key Difference Between rem and em in CSS?

CSS: The Difference Between Rem and Em

In CSS, developers often use various units to define the size of text elements. Two commonly used units are the rem and em. Though both units are similar, they differ in a crucial way.

Understanding the Rem Unit

The rem unit, short for root em, is relative to the root font size of the website. This means that the size of elements using rem will remain constant regardless of changes in font size within intermediate containers.

Understanding the Em Unit

In contrast to rem, the em unit is relative to the parent element's font size. When the parent element's font size changes, the size of elements using em will also adjust accordingly.

Demonstration

In the following CSS code:

div {
    font-size: 1.4rem;
}

div p {
    font-size: 1.4rem;
}

The div element is assigned a font size of 1.4rem, while the p element within the div is also assigned a font size of 1.4rem. If you were to increase the font size of the div element, the font size of the p element would not change because it is defined using rem, which is relative to the root font size.

Conversely, if you were to use em instead of rem, the font size of the p element would also increase when you increase the font size of the div element. This is because em is relative to the parent element's font size.

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