为什么Bootstrap4选择rem和em而不是px？-WhyBootstrap4chooseremandeminsteadpx?

17  

With rem, all font sizes are relative to the root element (aka, the html tag). The reason for this is to make it easier to scale up or down for devices. You could technically change the html tag to a smaller or larger size to scale all font sizes equally – which is a super nice feature.

使用rem，所有字体大小都相对于根元素（也就是html标记）。这样做的原因是为了更容易扩展或缩小设备。从技术上讲，你可以将html标签更改为更小或更大的尺寸，以平等地扩展所有字体大小 - 这是一个非常好的功能。

... [T]he main thing to take-away is everything is dynamic and relative to the root HTML tag.

... [T]他主要的东西是一切都是动态的，并且相对于根HTML标签。

For example, change the html css font-size to a different number ... and watch how the entire grid adjusts and scales.

例如，将html css字体大小更改为其他数字...并观察整个网格如何调整和缩放。

Source: Scotch.io

资料来源：Scotch.io

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It is worth mentioning that Bootstrap 4 kept breakpoints in px and not em. From the docs:

值得一提的是，Bootstrap 4在px而不是em中保留了断点。来自文档：

While Bootstrap uses ems or rems for defining most sizes, pxs are used for grid breakpoints and container widths. This is because the viewport width is in pixels and does not change with the font size.

虽然Bootstrap使用ems或rems来定义大多数大小，但px用于网格断点和容器宽度。这是因为视口宽度以像素为单位，并且不随字体大小而变化。