When selecting font size units, you should use px to fix the size; em to adjust the local relative size; rem to global control and responsive design. px is a fixed unit, suitable for borders, icons, and other scenes that do not require scaling; em is suitable for local styles such as button text relative to the parent element font size, but multi-layer nesting is prone to errors; rem is based on the root element html, which is convenient for unified control of the overall font size, and can realize responsive layout in conjunction with media queries. It is recommended for modern web development.

When using CSS to layout, should we use em, rem or px for font sizes and spacing? In fact, there is no standard answer, but you can choose a more suitable unit according to different scenarios. The key is to understand their respective characteristics and applicable conditions.

1. px: fixed size, simple and direct

px is the most basic unit, representing pixel points. Its advantage is intuitive - as much as you write, it will not be affected by the parent element or root element.

• Suitable for scenes where no scaling is required, such as border width, small icon size, etc.
• PX is also commonly used for positioning of certain fixed positions in the page layout, so errors are not prone to occur.

For example: If you set font-size: 16px for a button, it will maintain this size on all devices (unless the page is scaled by the user).

But the problem lies here too - it's too rigid. If you want the entire page to be adjusted according to the user's system settings, or to be responsively designed, px is not suitable.

2. em: relative to parent element font size

em is calculated relative to the font size of the parent element of the current element. For example, if the parent element is 16px , then if you write 1.5em , it is equal to 24px .

This sounds flexible, but it is also prone to errors, especially when the nesting is deep:

• Multi-layered em may cause font size to be enlarged or reduced layer by layer
• It is troublesome to maintain, especially when refactoring, it is not easy to see the final effect at a glance

So suggestion:

• Use em only in local styles, such as button internal text, card title, etc.
• Do not use em in global font settings, as it can easily cause chain reactions

3. rem: based on root element, more suitable for global control

rem is the font size relative to the element. By default, the browser's html font is 16px , so 1rem = 16px , 1.25rem = 20px , and so on.

This is the most recommended practice in modern web pages, especially for unified control of overall font size:

• Set the font-size of html once, and use rem to define it elsewhere
• Dynamically adjust the html font size with media queries or JS, which can easily achieve responsiveness

For example, you can write this:

 html {
  font-size: 16px;
}

@media (max-width: 768px) {
  html {
    font-size: 14px;
  }
}

In this way, all font sizes defined with rem will automatically change.

Let's summarize how to choose:

• Want to fix the size, don't change with user settings → Use px
• Need to adjust relative to parent, but only takes effect locally → use em
• I hope to control the whole world, responsively friendly → use rem

Basically these differences. Not complicated, but it's easy to get confused at the beginning. You can see which one is more suitable for the current project after using it a few times.

The above is the detailed content of Choosing between em, rem, and px units in css. For more information, please follow other related articles on the PHP Chinese website!