css drawing error

CSS Drawing "Errors"

In web design, CSS is an important tool. Through CSS, you can set the style of the page, such as font size, color, background, border, etc. However, with the continuous development of web technology, the functions of CSS are becoming more and more powerful. People can use CSS to achieve many effects that could only be achieved using technologies such as JavaScript or Flash. However, sometimes we also encounter some CSS pitfalls. Today let us discuss drawing "errors" in CSS.

1. Margin and padding in the box model

In CSS, the box model is a very common and important concept. The box model consists of four parts: the content area, padding, borders, and margins of the element. Among them, margin and padding are two properties that control the outer and inner spacing of the box. However, their use is also prone to errors.

For example, in a page containing divs, if we want to have a certain distance between the div tags and the page, we can usually use margin:

div {
  margin: 20px;
}

However, if we are at this time If a p tag is added to the div tag, the margin of the p tag will be superimposed on the margin of the div, causing the spacing to be larger than expected.

At this time, the correct approach is to set padding instead of margin for the div:

div {
  padding: 20px;
}

In this way, the margin of the p tag will be superimposed inside the padding of the div instead of on the margin of the div. Overlay to achieve the effect we want.

2. Calculation of width

In CSS, the width is calculated through the content-box of an element. That is, if a div element is 500px wide, then its content-box will also be 500px wide. However, if the div element has a 5px border, then the div element will be 10px wider than expected.

This problem can be solved by adding the box-sizing attribute to the element. If box-sizing is set to border-box, the width includes the border and padding, not just the content-box. As shown below:

div {
  width: 500px;
  border: 5px solid black;
  box-sizing: border-box;
}

3. z-index property

In CSS, the z-index property allows an element to cover other elements on the z-axis (depth). However, sometimes we find that the effect of z-index is not as expected.

For example, in the following code, we expect that the content of #box2 will overwrite the content of #box1:

<div id="box1"></div>
<div id="box2"></div>

#box1 {
  position: relative;
  z-index: 1;
  background-color: red;
}
#box2 {
  position: relative;
  z-index: 2;
  background-color: blue;
}

However, when we actually ran this code, we found that #box2 was not completely Cover #box1. This is because both #box1 and #box2 are relatively positioned elements and they are in the same document flow. Therefore, their hierarchical relationship is actually determined by the order in which they appear in the HTML code. That is to say, although #box2 has a larger z-index than #box1, it does not completely cover #box1.

If we want #box2 to completely cover #box1, then we can turn #box1 into an absolutely positioned element:

#box1 {
  position: absolute;
  z-index: 1;
  background-color: red;
}
#box2 {
  position: relative;
  z-index: 2;
  background-color: blue;
}

In this way, #box1 is no longer an element in the document flow, Rather, it is an element positioned relative to the parent element (that is, the body element), so that the z-index of #box2 can completely cover #box1.

Summary

CSS is a very powerful tool, but it is inevitable to encounter some pitfalls in the process of using it. This article covers some common CSS mistakes, including margin and padding in the box model, width calculations, and the use of the z-index property. Understanding these common mistakes can allow us to avoid unnecessary mistakes in daily development and improve work efficiency.

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