flex-flow作用：flex-direction和flex-wrap属性的简写形式。
语法：flex-flow: flex-direction flex-wrap
示例：
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><style>#div0{width:300px;background-color: violet;display:flex;flex-flow:row wrap;}#div0 div{width: 100px;height: 100px;background-color: yellow;}</style></head><body><div id="div0"><div>1</div><div>2</div><div>3</div><div>4</div></div></body></html>
