vue中设定html颜色格式报错：[Vue warn]: Failed to resolve directive: html:

    <div id="app-2">
        <p>
            using: {{ message }}
        p>
//这个是报错原因，已注释
        <p>this will be blue: <span v-html="message" style="color: blue">span> p>
        <p>this will be red: <span v-html="message" style="color: red">span> p>
    div>
    <script>
         'use strict'
        let app2 = new Vue({
            el: '#app-2',
            data: {
                message: 'html style'
            }
        })
    script>

　　显示结果：

using: html style

this will be blue: html style

this will be red: html style