Controlling inheritance

CSS provides four special universal property values for controlling inheritance：

1. inherit ： to be the same as that of its parent element
2. initial ： to the initial value of that property
3. unset ： acts like inherit, otherwise it acts like initial
4. revert ： new feature

The CSS shorthand property all can be used to apply one of these inheritance values to (almost) all properties at once.
It's a convenient way to undo changes made to styles so that you can get back to a known starting point before beginning new changes.

Specificity

The amount of specificity a selector has is measured using four different values (or components), which can be thought of as thousands, hundreds, tens and ones — four single digits in four columns.

1. **Thousands**: Score one in this column if the declaration is inside a style attribute, aka **inline styles**. Such declarations don't have selectors, so their specificity is always simply 1000.
   

2. **Hundreds**: Score one in this column for each **ID selector** contained inside the overall selector.
   

3. **Tens**: Score one in this column for each **class selector**, **attribute selector**, or **pseudo-class** contained inside the overall selector.
   

4. **Ones**: Score one in this column for each **element selector** or **pseudo-element contained inside the overall selector**.
   

Note: The universal selector (*), combinators (+, >, ~, ' '), and negation pseudo-class (:not) have no effect on specificity.

!important

仅仅用在非常特殊的情况，可以强制修改前面所描述所有规则。
用法：border: none !important;

如果两个规则都用了！importance则对比规则的先后顺序和specificity

仅仅作为一种知识的补充，但是并不推荐大家使用这个方法，除非当你无法修改CSS代码的时候可以考虑。