Sizing items in CSS

在前面的课程中你已经看到了几种使用CSS为页面中元素设定尺寸的方法。 在我们设计网页的时候,需要理解这些不同方法之间的差异。在本课程中,我们将总结设定元素尺寸的方法,并定义几个术语,这些内容将会在未来对你有所帮助。

Prerequisites: Basic computer literacy, basic software installed, basic knowledge of working with files, HTML basics (study Introduction to HTML), and an idea of how CSS works (study CSS first steps.)
Objective: To understand the different ways we can size things in CSS.



如果你把图片放置在网页中的时候没有在<img> 标签或CSS中设置其尺寸,那么将使用其固有尺寸显示。我们给下面的示例图像绘制了一个边框,以便你看出图像文件的长宽。

一个空的<div>是没有尺寸的。如果你在你的HTML文件中添加一个空<div> 并给予其边框(就像刚才我们为图像做的那样),你会在页面上看到一条线。这是边框被压缩后的效果— 它内部没有内容。在我们下面的例子中,边框宽度扩展到整个容器宽度,因为它是块级元素,而块级元素的行为就是这样的。它没有高度,或者说高度为0,因为内部没有内容。

在上面的例子中,试着在空元素内部添加些内容。现在边框内包含一些文字了,因为元素的高度由其所含内容高度确定。再强调一次,这就是元素的固有尺寸 — 由其所包含的内容决定。

Setting a specific size

We can of course give elements in our design a specific size. When a size is given to an element (and the content of which then needs to fit into that size) we refer to it as an extrinsic size. Take our <div> from the example above — we can give it specific width and height values, and it will now have that size no matter what content is placed into it. As we discovered in our previous lesson on overflow, a set height can cause content to overflow if there is more content than the element has space to fit inside it.

Due to this problem of overflow, fixing the height of elements with lengths or percentages is something we need to do very carefully on the web.

Using percentages

In many ways percentages act like length units, and as we discussed in the lesson on values and units, they can often be used interchangeably with lengths. When using a percentage you need to be aware what it is a percentage of. In the case of a box inside another container, if you give the child box a percentage width it will be a percentage of the width of the parent container.

This is because percentages resolve against the size of the containing block. With no percentage applied our <div> would take up 100% of the available space, as it is a block level element. If we give it a percentage width, this becomes a percentage of the space it would normally fill.

Percentage margins and padding

If you set margins and padding as a percentage you may notice some strange behavior. In the below example we have a box. We have given the inner box a margin of 10% and a padding of 10%. The padding and margin on the top and bottom of the box are the same size as the margin on the left and right.

You might expect for example the percentage top and bottom margins to be a percentage of the element's height, and the percentage left and right margins to be a percentage of the element's width. However, this is not the case!

When you use margin and padding set in percentages, the value is calculated from the inline size — therefore the width when working in a horizontal language. In our example, all of the margins and padding are 10% of the width. This means you can have equal sized margins and padding all round the box. This is a fact worth remembering if you do use percentages in this way.

min- and max- sizes

In addition to giving things a fixed size, we can ask CSS to give an element a minimum or a maximum size. If you have a box that might contain a variable amount of content, and you always want it to be at least a certain height, you could set the min-height property on it. The box will always be at least this height, but will then grow taller if there is more content than the box has space for at its minimum height.

In the example below you can see two boxes, both with a defined height of 150 pixels. The box on the left is 150 pixels tall; the box on the right has content that needs more room, and so it has grown taller than 150 pixels.

This is very useful for dealing with variable amounts of content while avoiding overflow.

A common use of max-width is to cause images to scale down if there is not enough space to display them at their intrinsic width, while making sure they don't become larger than that width.

As an example, if you were to set width: 100% on an image, and its intrinsic width was smaller than its container, the image would be forced to stretch and become larger, causing it to look pixellated. If its intrinsic width were larger than its container, it would overflow it. Neither case is likely to be what you want to happen.

If you instead use max-width: 100%, the image is able to become smaller than its intrinsic size, but will stop at 100% of its size.

In the example below we have used the same image twice. The first image has been given width: 100% and is in a container which is larger than it, therefore it stretches to the container width. The second image has max-width: 100% set on it and therefore does not stretch to fil the container. The third box contains the same image again, also with max-width: 100% set; in this case you can see how it has scaled down to fit into the box.

This technique is used to make images responsive, so that when viewed on a smaller device they scale down appropriately. You should however not use this technique to load in really large images and then scale them down in the browser. Images should be appropriately sized to be no larger than they need to be for the largest size they are displayed in the design. Downloading overly large images will cause your site to become slow, and it can cost users more money if they are on a metered connection.

Note: Find out more about responsive image techniques.

Viewport units

The viewport — which is the visible area of your page in the browser you are using to view a site — also has a size. In CSS we have units which relate to the size of the viewport — the vw unit for viewport width, and vh for viewport height. Using these units you can size something relative to the viewport of the user.

1vh is equal to 1% of the viewport height, and 1vw is equal to 1% of the viewport width. You can use these units to size boxes, but also text. In the example below we have a box which is sized as 20vh and 20vw. The box contains a letter A, which has been given a font-size of 10vh.

If you change the vh and vw values this will change the size of the box or font; changing the viewport size will also change their sizes because they are sized relative to the viewport. To see the example change when you change the viewport size you will need to load the example in a new browser window that you can resize (as the embedded <iframe> that contains the example shown above is its viewport). Open the example, resize the browser window, and observe what happens to the size of the box and text.

Sizing things according to the viewport can be useful in your designs. For example, if you want a full page hero section to show before the rest of your content, making that part of your page 100vh high will push the rest of the content below the viewport, meaning that it will only appear once the document is scrolled.


This lesson has given you a rundown of some key issues that you might run into when sizing things on the web. When you move onto CSS Layout, sizing will become very important in mastering the different layout methods, so it is worth understanding the concepts here before moving on.


  1. 级联和继承
  2. CSS选择器
  3. 盒模型
  4. 背景和边框
  5. 文字方向排版
  6. 内容溢出
  7. 值和单位
  8. CSS中的大小调整项
  9. 图像、媒体和表单元素
  10. 样式表
  11. CSS调试
  12. 组织CSS代码