让我们回来的文档，人们的答案。
参考：CSS 2.1: 4.3.2 Lengths
“The format of a length value (denoted by <length> in this specification) is a <number> (with or without a decimal point) immediately followed by a unit identifier (e.g.,px,em,etc.).”
当元素显示在屏幕上时，大多数浏览器自然会将位置四舍五入到最接近的像素。
