How to use rem in css3

rem is a relative size value, relative to the unit of html element font size, and the syntax format is "element: number rem". rem changes the font size of the browser, which will break the layout of our web page.

The operating environment of this tutorial: Windows 7 system, CSS3&&HTML5 version, Dell G3 computer.

css3 has added a new relative unit rem. Using rem is the same as em as a relative font size unit. It is a relative unit, relative to the unit of the root element font size. To put it more bluntly, it is relative to the font size of the html element. The unit.

Advantages: When calculating the size of sub-elements, you only need to calculate it based on the font size of the html element. It is no longer like when using em, you have to go back and forth to find the font size of the parent element for frequent calculations, and you cannot do without a calculator at all.

rem is the unit

With the emergence of CSS3, he also introduced some new units, including what we call rem today. Rem is described on the W3C official website like this - "font size of the root element". Let's take a closer look at rem.

rem is relative to the root element , which means that we only need to determine a reference value in the root element and set the font size in the root element. This can be completely based on your own needs. , you can also refer to the picture below:

Let’s look at a simple example:

html {font-size: 62.5%;/*10 ÷ 16 × 100% = 62.5%*/}
body {font-size: 1.4rem;/*1.4 × 10px = 14px */}
h1 { font-size: 2.4rem;/*2.4 × 10px = 24px*/}

I defined a in the root element The basic font size is 62.5% (that is, 10px. Setting this value is mainly to facilitate calculation. If it is not set, it will be based on "16px"). From the above calculation results, we can use "rem" as conveniently as "px", and at the same time solve the difference between "px" and "em".

Use of rem

First we need to add this tag:

<meta name="viewport" content="initial-scale=1,maximum-scale=1, minimum-scale=1">

The specific meaning is as follows:

initial-scale - the initial scaling ratio minimum-scale - The minimum ratio that the user is allowed to zoom to maximum-scale - The maximum ratio that the user is allowed to zoom to

Then set the font-size of the html:

// resize 窗口大小发生改变的时候才会触发的，第一次加载时不会触发
var resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
    // 手机屏幕是否反转orientationchange
    window.addEventListener(resizeEvt, setFontSize, false); // IE 谷歌
document.addEventListener('DOMContentLoaded', setFontSize, false); //火狐
function setFontSize() {
    var cWidth = window.innerWidth || document.documentElement.clientWidth || document.body.clientWidth;
    var basicNum = 200;
    var htmlFontSize = basicNum * (cWidth / 设计稿宽度);
    document.documentElement.style.fontSize = htmlFontSize + 'px';
}

The width of the general design draft is 750, in During the development process, we used iPhone 6/7/8 as the benchmark, so the font-size of the HTML should be 200*(375/750)=100px; so if there is a box with a width of 200px in the design draft. We need to set his width to (200/100)rem; so this is why I recommend setting basicNum to 200, because it is simple to calculate.

var htmlFontSize=basicNum*(cWidth/设计稿宽度);

It can be seen that the wider the screen size, the larger the font-size of the root html. Since other elements are

based on it, they can be adaptive.

Browser compatibility

rem is a new measurement unit introduced by CSS3. Everyone will definitely feel frustrated and worried about browser support. In fact, there is no need to be afraid. You may be surprised that there are quite a few supported browsers, such as: Mozilla Firefox 3.6, Apple Safari 5, Google Chrome, IE9 and Opera11.

However, when using units to set fonts, you cannot completely ignore IE. If you want to use this REM, but also want to be compatible with the effect under IE, you can consider using "px" and "rem" together. Use "px" to achieve the effect under IE6-8, and then use "Rem" to achieve the effect of the browser.

Recommended learning: css video tutorial

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