Value: [
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
由border-width，border-style，border-color，这三个部分组成，其顺序随意。
但为了代码的可读性，建议按照，border-width，border-style，border-color，的顺序书写。
