ID>CLASS>LABLE

1em === recently pounds of word, its parient  is the closest html attribute

rem : root em

a good default font size: 

0.875em === 14px

border ： default box-sizing = content-box， it will just set w and h of content-box , the box model will not change. if set box-sizing = border-box, you can change height and width.

background :

        background-image

        background-position

        background-size

        background-repeat

        background-origin

        background-clip

        background-attachment

of course background-color

The function : linear-gradient()

The linear-gradient function have three parameters that angle, start-color, end-color.

the angle, you can choose diferent parameters, such as to bottom right, to bottom, to top.

If you want to have a "color stop", you can directly add it to mid node: 

        linear-gradient (to right, red 0%, white 50%, blue 100%)

function: repeat-linear-gradient()

box-shadow()

        box-shadow can give element box a shadow mode, 

        text-shadow can give text a shadow mode

1em means "move number" horizontal direction first，vertical direction two step

if "move number" all are 0 , the shadow will be showed beneath  the element.

black is shadow color.

at the default situation, shadow and element size is completely same. if the element have a radius angle, the shadow too.

3d effect

:active{ }