CSS3-條件判斷（@supports）

@supports (property: value){
  element {
    property: value;
  }
}

.flex {
  float: left;
}

@supports (display: flex){
  .flex {
    display: flex;
    float: none;
  }
}

@supports not (property: value){
  element {
    property: value;
  }
}

@supports (property: value) and (property: value) {
  element {
    property: value;
  }
}

@supports (property: value) or (property: value) {
  element {
    property: value;
  }
}

@supports ((property: value) or (property: value)) and (property: value) {
  element {
    property: value;
  }
}

var supportsCSS = !!((window.CSS && window.CSS.supports) || window.supportsCSS || false); 

CSS.supports(property, value)

CSS.supports('property: value');