By default, flex items won’t shrink below their minimum content size

flex 布局的元素会默认设置min-widthmin-width默认值为 0，但规范中对于flex项目则设置为auto。这可能会使块状元素占用比预期更多的空间，甚至使它们的容器超出小屏幕的屏幕边缘。

#### 参考阅读：

For width/height, specifies an automatic size. See the relevant layout module for how to calculate this.
For min-width/min-height, specifies an automatic minimum size. Unless otherwise defined by the relevant layout module, however, it resolves to a used value of 0. For backwards-compatibility, the resolved value of this keyword is zero for boxes of all [CSS2] display types: block and inline boxes, inline blocks, and all the table layout boxes. It also resolves to zero when no box is generated.