1. comment：
HTML:<!--comment-->
CSS: /*comment*/
2. em:
HTML: 斜体
CSS:font size; one em is equal to the default font size on whatever screen the user is using.
3. self closing tab:
<img src="XXXXX"/>
<link type="xxx" rel="xxx" href="xxx"/>
4. .class: for punch of sectors
#ID: for exactly one sector.
5. 想让div变成circle，设置radius为100%
6.
Miscellaneous：
1. css里面properties：后面space
2. css里面<a href="xxxx>默认的text decoration是蓝色带下划线，可设置为text decoration:none
3. 取色16进制：（0~9）（A~F）#XXXXXX
4. 如果同一个selector既设置了class又设置了id，同步执行；如果两者value冲突，执行后面一个。
