CSS 单位
em and rem： are often used to create scalable layouts, which maintain the vertical rhythm of the page even when the user changes the font size.
cap：Represents the "cap height" (nominal height of capital letters) of the element’s font.
ch：Represents the width, or more precisely the advance measure, of the glyph "0" (zero, the Unicode character U+0030) in the element's font。
em：Represents the calculated font-size of the element. If used on the font-size property itself, it represents the inherited font-size of the element.
ex：Represents the x-height of the element's font. On fonts with the "x" letter, this is generally the height of lowercase letters in the font; 1ex ≈ 0.5em in many fonts.
ic：Equal to the used advance measure of the "水" (CJK water ideograph, U+6C34) glyph found in the font used to render it.lhEqual to the computed value of the line-height property of the element on which it is used, converted to an absolute length.
rem：Represents the font-size of the root element (typically <html>). When used within the root element font-size, it represents its initial value (a common browser default is 16px, but user-defined preferences may modify this).
rlh:Equal to the computed value of the line-height property on the root element (typically <html>), converted to an absolute length. When used on the font-size or line-height properties of the root element, it refers to the properties' initial value.
vh:Equal to 1% of the height of the viewport's initial containing block.
vw:Equal to 1% of the width of the viewport's initial containing block.
vi:Equal to 1% of the size of the initial containing block, in the direction of the root element’s inline axis.
vb:Equal to 1% of the size of the initial containing block, in the direction of the root element’s block axis.
vmin:Equal to the smaller of vw and vh.
vmax:Equal to the larger of vw and vh.
px:One pixel. For screen displays, it traditionally represents one device pixel (dot). However, for printers and high-resolution screens, one CSS pixel implies multiple device pixels. 1px = 1/96th of 1in.
cm:One centimeter. 1cm = 96px/2.54.
mm:One millimeter. 1mm = 1/10th of 1cm.
Q:One quarter of a millimeter. 1Q = 1/40th of 1cm.
in:One inch. 1in = 2.54cm = 96px.
pc:One pica. 1pc = 12pt = 1/6th of 1in.
pt:One point. 1pt = 1/72nd of 1in.