你可以使用css三维变换
比如

input {
    width: 280px;
    -webkit-transition: width 1s ease;
    -moz-transition: width 1s ease;
    -o-transition: width 1s ease;
    -ms-transition: width 1s ease;
    transition: width 1s ease;
}

input:focus {
    width: 340px;
}