Selecting the right CSS unit in web design can improve layout flexibility and responsiveness. px is suitable for precise control, such as border or icon size; em is based on the current font size, used for proportional layout but is susceptible to nesting; rem is based on the root element, suitable for global unified design; vw/vh is based on the viewport size, suitable for full-screen layout; % is relative to the parent container, suitable for width or margin settings. In responsive design, rem is used to achieve overall scaling, and combining %, fr or clamp() functions can enhance adaptability and visual effects. Avoid mixing em and rem, use px to set fonts with caution, and recommend unifying rem to improve maintainability. By reasonably matching different units, a more flexible and consistent layout plan can be achieved.

In web design, choosing the right CSS unit is a key part of achieving scalable and responsive layouts. Many people like to use px when writing styles at the beginning, but in fact, using different units in different scenarios can bring better flexibility and adaptability.

Understand the applicable scenarios of different units

CSS provides a variety of units of length, such as px , em , rem , vw , vh and % , each with its own purpose.

• px (pixels) is the most basic unit, it is fixed and suitable for occasions where precise control is required, such as border width or icon size.
• em is a unit relative to the font size of the current element. It is often used to build a layout system with consistent proportions, but it will inherit the influence of the parent element and easily cause confusion during nesting.
• rem is a unit based on the font size of the root element (usually ) and is more suitable for global unified design and is not prone to errors.
• vw / vh represents the percentage units of viewport width and height, respectively, 1vw = 1% of viewport width, suitable for layouts of full-screen display classes.
• % is often used in scenarios such as width and margins that are set relative to the parent container.

Only after understanding the characteristics of these units can you make reasonable choices based on specific needs.

Flexible use of relative units in responsive design

Responsive design emphasizes "adaptation", that is, the page can be displayed well on different devices. At this time, the absolute unit (such as px) is not enough, and the advantages of the relative unit are reflected.

• Use rem to set the font size, so that the text of the entire page can be adjusted with the root element, making it easier to scale the overall.
• Defining the layout structure using fr unit in % or flex / grid can achieve automatic scaling column width and row height.
• Combining vw can automatically enlarge the title or picture on the large screen to enhance the visual impact, but be careful not to make it too small or too large. You can use clamp() to limit the range.

For example:

 .title {
  font-size: clamp(1.5rem, 5vw, 3rem);
}

In this way, no matter how big the screen is, the font will not exceed the set minimum and maximum values.

Avoid common pitfalls and keep consistency

Although there are various units available, some common misunderstandings should be paid attention to:

• Don't mix em and rem too often, especially in deep nested structures, it is easy to be confused about the actual calculated values.
• Try to avoid using px for font size, especially on mobile, users may set higher default font sizes.
• For spacing and container size, rem is preferred, so that the page is still friendly when the user changes the browser font size.
• If you are working on a system that supports theme switching or dynamic scaling, then using rem is a more controllable way.

A small trick: You can set html { font-size: 62.5%; } in the root element, so that 1rem equals 10px, and the numbers are easier to convert.

Basically that's it. Choosing the right unit is not complicated, but it is easy to be ignored. Using it according to project type, design style and response needs can make the layout more flexible and maintainable.

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