How to maintain the aspect ratio of page content using CSS

This article mainly introduces how to use CSS to maintain the aspect ratio of page content. The author gives examples of different methods such as pseudo elements and vw units. Friends in need can refer to the following

Requirement description : The mobile terminal implements a semicircle across the page. (Similar problem, implementing a 4x4 square grid)

# Simplifying the problem, we can understand it as implementing a block with a height and width ratio of 1:2.

Problems need to be solved:

　　　1. The height and width are in a certain proportion.

　　　2. The height and width of the outer container are uncertain.

3. Try not to use pictures and scripts instead.

　　　4. Compatible with mobile terminals.

Writing html

<p class = "semicircle"></p>

Think one, use height: 100%,

body{   
    margin:0;   
    width: 100%;   
    background: lightblue;   
}   
.semicircle {   
    width: 100%;   
    height: 100%;   
    border-top:5px solid #fff;   
    border-radius: 100%;   
}

There is a problem, the percentage of height is calculated based on the parent container, It is not the current container and cannot meet our needs at all. The effect is as follows:

The height percentage of the parent container body is related to the height filled by its child container. Even if the body height is set to 100%, because the child container is the semicircle The actual height of the padding is 5 of the border, and the parent container cannot be "fully expanded". Therefore, it is impossible to specify a container with a certain proportion of width and height by setting the height of the parent container as a percentage.

Thinking 2, set padding-top or padding-bottom to 100%
The percentage is calculated with respect to the width of the generated box's containing block [...] (source: w3.org, emphasis mine)

The calculation of the percentage width is related to the width of the containing block of the generated box. The percentages of padding-top and padding-bottom are calculated based on the width of the parent container, not the height. Other ratio implementation comparison table

##8:562.5%

body{   
    margin:0;   
    width: 100%;   
    background: lightblue;   
}   
.semicircle {   
    width: 100%;   
    height: 0;   
    padding-bottom: 100%;   
    border-top:5px solid #fff;   
    border-radius: 100%;   
}

aspect ratio	padding-bottom value
16:9	56.25%
4:3	75%
3:2	66.66 %

Thinking three, using vw unit
Use the vw unit to set the height and width of the element. The size of the vm is set by the width of the viewport, so this method can be used to keep the container displayed at a certain proportion. One unit of vw is equal to one percent of the viewport width, that is, 100vw is equal to 100% viewport width.

body{   
    margin:0;   
    width: 100%;   
    background: lightblue;   
}   
.semicircle {   
     width: 100vw;   
      height:100vw;   
    border-top:5px solid #fff;   
    border-radius: 100%;   
}

Comparison table

aspect ratiomultiply width by 1:11##1:34:316:9##Thinking 4, use pseudo elements And inline-block layout

3
0.75
0.5625

body {   
    width: 100%;   
    font-size: 0;   
    text-align: center;   
    background: lightblue;   
}   
.semicircle {   
    border-top:5px solid #fff;   
    border-radius: 100%;   
}   
.semicircle:before {   
    content:"";   
    display: inline-block;   
    padding-bottom: 100%;   
}

Although the code is a bit complicated, it is highly flexible and can achieve more similar effects.

When the requirement is changed to implement a semicircle spanning 80% of the width of the screen, we only need to add the attribute width:80%; in .semicircle, and also center the container.

The principle of this method is very clear:

Refer to Thought 1, if the height of the outer container cannot be expanded by 100%, then you can insert an element with the same height and width through a pseudo element, and The container is stretched to a height of one to one. Note that this method to implement a semicircle actually requires a container with a width and height ratio of one to one, that is, it takes up twice as much space as the above method.

Settings: before element boundary, parsing principle:

Thinking five, use pictures, compatible with low-end mobile devices.

.semicircler img {   
  width: 100%;   
  background-repeat: no-repeat;   
  background-size: 100% 100%;   
  background-image: url(../img/autoresized-picture.jpg);   
}

Using scripts, css is more concise and clear, and the goal is clear.

p.style.height=p.offsetWidth+"px";

For the realization of 2*2 square grid

*------main code-------*/   
        body {   
          width: 100%;   
          margin:0;   
          text-align: center;   
        }   
        p{   
          display: inline-block;   
          width: 50%;   
          background: lightblue;   
          font-size: 12px;   
          position: relative;   
          vertical-align: middle;   
        }   
        p:before {   
            content:"";   
            display: inline-block;   
            padding-bottom: 100%;   
            vertical-align: middle;   
        }   
        /*------other code-------*/
        p:nth-child(2),p:nth-child(3){   
            background: pink;   
        }   
        span {   
            display: inline-block;   
            vertical-align: middle;   
            font-size: 6em;   
            color: #fff;   
        }

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