Common Line box: contains the specific content elements
span、strong、em、i、img、video、audio
Notable features:
1. box extends content
2. The line width cartridge can not be set high, adjusting the line width and height of the box, use the font size, line height, font type, indirect adjustment
3. Padding (padding area), borders, margins
Effective horizontal direction, the vertical direction only affects the background, not actually occupy space
