The choice of CSS units depends on design requirements and responsive requirements. 1. px is used for fixed size, suitable for precise control but lack of elasticity; 2. em is a relative unit, which is easily caused by the influence of the parent element, while rem is more stable based on the root element and is suitable for global scaling; 3. vw/vh is based on the viewport size, suitable for responsive design, but attention should be paid to the performance under extreme screens; 4. When choosing, it should be determined based on whether responsive adjustments, element hierarchy relationships and viewport dependence. Reasonable use can improve layout flexibility and maintenance.

In web design, choosing the right CSS unit is crucial to layout and responsive design. Although px, em, rem, vw, and vh are common units, their usage scenarios and behaviors are very different.

1. px: a representative of fixed size

px (pixel) is the most basic and intuitive unit, which represents a point on the screen. Using px ensures that elements maintain consistent display on different devices, suitable for designs that require precise control. However, the disadvantage of px is its fixedness and cannot be adjusted according to the user's settings or viewport size. For example, after setting the font size to font-size: 16px; the text size will not change regardless of whether the user adjusts the browser's zoom. This "crazy" feature may affect the user experience in some cases, especially on mobile.

2. em and rem: flexibility of relative units

em is a relative unit whose value is relative to the font size of the current element. If the font size of the parent element changes, the em value of the child element will also change. This can lead to cascading problems, such as in a nested structure, multiple levels of em calculations can make styles unpredictable. Therefore, special attention should be paid to the context when using em.

In contrast, rem (root em) is more stable. It is calculated based on the font size of the HTML root element and is usually 16px by default. This means that no matter how deep the element is nested, as long as the font size of the root element remains unchanged, the performance of rem is consistent. rem is particularly suitable for unified control of global font size, such as dynamically adjusting the font size of the root element through JavaScript to achieve the overall page scaling effect.

3. vw and vh: Responsive Advantages of Viewport Units

vw (viewport width unit) and vh (viewport height unit) are units based on the browser viewport size. 1vw is equal to 1% of the viewport width, and 1vh is equal to 1% of the viewport height. These two units are great for creating responsive designs as they automatically adjust as the screen size changes. For example, setting the height of a full-screen background image to height: 100vh; ensures that it always occupies the entire screen height without being affected by other elements.

However, vw and vh also have some things to note:

• On mobile devices, the address bar of some browsers may dynamically affect viewport height, resulting in unstable vh behavior.
• When using vw to set the font size, there may be problems with too large or too small fonts, especially on ultra-wide screens.

4. How to choose the right unit?

The key to choosing a CSS unit is to understand the design requirements and the characteristics of the target equipment:

• If you are pursuing absolute precise control without responsive adjustments, px is a reliable choice.
• Rem is more recommended than em for text or components that need to be scaled because it avoids the complexity of cascade.
• vw and vh are ideal when you need elements to automatically adjust with viewport changes, but be careful to handle performance in extreme cases.
• Certain modern units such as ch (character width), ex (x letter height), etc. also have specific uses, but are used less frequently in daily development.

Basically, this is all. Using these units reasonably can make your layout more flexible and easier to maintain.

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