文本大小：{font-size:12px/14px/16px;}
说明：
1）属性值为数值型时，必须给属性值加单位，属性值为0时除外。
2）单位还可以是pt，9pt=12px;
3）为了减小系统间的字体显示差异，IE Netscape Mozilla的浏览器制作商于1999年召开会议，共同确定16px/ppi为标准字体大小默认值,即1em.默认情况下，
0.75em=12px;
1em=16px;
4)使用绝对大小关键字
xx-small =9px
x-small =11px
small =13px
medium =16px
large =19px
x-large =23px
xx-large =27px
5)具体案例：
1in = 2.54cm = 25.4 mm = 72pt = 6pc
Points Pixels Ems Percent
6pt 8px 0.5em 50%
7pt 9px 0.55em 55%
7.5pt 10px 0.625em 62.5%
8pt 11px 0.7em 70%
9pt 12px 0.75em 75%
10pt 13px 0.8em 80%
10.5pt 14px 0.875em 87.5%
11pt 15px 0.95em 95%
12pt 16px 1em 100%
13pt 17px 1.05em 105%
13.5pt 18px 1.125em 112.5%
14pt 19px 1.2em 120%
14.5pt 20px 1.25em 125%
15pt 21px 1.3em 130%
16pt 22px 1.4em 140%
17pt 23px 1.45em 145%
18pt 24px 1.5em 150%
20pt 26px 1.6em 160%
22pt 29px 1.8em 180%
24pt 32px 2em 200%
26pt 35px 2.2em 220%
27pt 36px 2.25em 225%
28pt 37px 2.3em 230%
29pt 38px 2.35em 235%
30pt 40px 2.45em 245%
32pt 42px 2.55em 255%
34pt 45px 2.75em 275%
36pt 48px 3em 300%
