HTML&CSS

1. comment：

    HTML:

    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:

   

   

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