If you are passing this task: create a silver-backgroud class and make backgroud color of div become silver,
and you find you have written a correct code,but it did’t work,
you need to check your input method:
or you just use cntrl+c and cntrl+v rather than use your input method to tap the code.
I just wanna express sometime the input method maybe incompatible.（输入法可能不兼容）