Improve user experience skills and writing project skills

1. Reduce user operations

1. Add a radio button to be selected by default

2. Project Writing Skills

1. Leave two spaces blank at the beginning: Use text-indent:2em;
   because 1em == 1 front-size , front-size is the height of the font, but now for most Chinese fonts, the font width is equal to the font height , so leave 2em blank at the beginning, no matter how many fonts are set Size, there will be two empty spaces.
2. absolute and relative
   absolute: positioning away from the original position, positioning according to the nearest positioned parent, if there is none, positioning relative to the document relative
   : retaining the original position for positioning, positioning relative to its own original position
   . Therefore, When used, generally parent elements use relative positioning and child elements use absolute positioning.