em:
相对于应用在当前元素的字体尺寸,1em 等于当前的字体尺寸,2em 等于当前字体尺寸的两倍,一般浏览器字体大小默认为16px，则2em == 32px；
W3原文：font size of the element；Equal to the computed value of the font-size property of the element on which it is used.
ex：
一个 ex 是一个字体的 x-height。 (x-height 通常是字体尺寸的一半。)
W3原文：x-height of the element’s font；Equal to the used x-height of the first available font [CSS3-FONTS]. The x-height is so called because it is often equal to the height of the lowercase "x". However, an ex is defined even for fonts that do not contain an "x". The x-height of a font can be found in different ways. Some fonts contain reliable metrics for the x-height. If reliable font metrics are not available, UAs may determine the x-height from the height of a lowercase glyph. One possible heuristic is to look at how far the glyph for the lowercase "o" extends below the baseline, and subtract that value from the top of its bounding box. In the cases where it is impossible or impractical to determine the x-height, a value of 0.5em must be assumed.
rem:
根元素（html）的 font-size.
W3原文：font size of the root element，Equal to the computed value of font-size on the root element.
If used in the font-size property of the root element, or in a document with no root element, 1rem is equal to the initial value of the font-size property.
ch:
数字 0 的宽度;
W3原文：Equal to the used advance measure of the "0" (ZERO, U+0030) glyph found in the font used to render it. (The advance measure of a glyph is its advance width or height, whichever is in the inline axis of the element.)
%:
A keyword matching one of the <length>, <angle>, <time>, or <frequency> units The attribute value must parse as a <number-token>, and is interpreted as a dimension with the specified unit. The default is 0 in the relevant units, or else the property’s minimum value if 0 in the relevant units is not valid for the property. The default must also be used if the property in question only accepts values within a certain range (e.g. positive lengths or angles from 0 to 90deg) and the attribute is out of range (e.g. a negative length or 180deg). If the unit is a relative length, it must be computed to an absolute length.
fr:
fr (fraction)单位是一个自适应单位，fr单位被用于在一系列长度值中分配剩余空间，如果多个已指定了多个部分，则剩下的空间根据各自的数字按比例分配。
数据来源：https://www.w3.org/TR/2018/CR-css-values-3-20180814/#intro