When I was writing styles of affix, whose width needs to be set by the calc function of CSS

.fix {
  position: fixed;
  top: 20px;
  width: calc(100%-20px);

}

However, the width is not working and the calc function fail to calculate the resulting px. After searching for this problem, I finally found out that the calc function should be written like the following

.fix {
  position: fixed;
  top: 20px;
  width: calc(100% - 20px);

}

That is, two whitespaces need to be reserved before and after the arithmatic operator.