All CSS properties involving lengths should be handled with caution. We recommended that you use proportional units (em, %) instead of absolute units (px) whenever possible. Using the former allows the UI to scale proportionately depending on font size settings, which are often changed by users with accessibility limitations. A common exception to this rule is when your CSS is directly related to images, where you'll usually handle measurements in pixels (px).

所有的涉及长度（width，height等）必须小心的使用。我们建议你尽可能使用成比例的单位（em，%）来代替绝对的单位（px）。使用前者允许UI根据字体尺寸的设置来按比例改变，因为字体尺寸会经常被用户改变。

 

Also, just like in HTML, you can control the dimensions of inflexible elements using the and  CSS properties and attributes. These attributes won't have any effect on flexible elements.

你也可以向HTML一样用css的width和height属性来设置元素的宽度和高度 ，但是这些属性不会对添加了flex属性的元素起作用