样式

<style>
        .itp {
            width: 200px;
            height: 60px;
            border: 5px solid red;
            text-align: center;
            line-height: 60px;
            font-size: 30px;
            margin: 10px;
        }

        button {
            width: 80px;
            height: 30px;
            color: rgb(229, 225, 232);
            border-radius: 4px;
            font-size: 20px;
            outline: none;
            border: none;
            background-color: blueviolet;
            /* margin-top: 10px; */
        }

      
    </style>

html

 <div>
        <div class="itp">a</div>
        <button class="ks">开始</button>
        <button class="tz">停止</button>
        <div>你的选择是：<span></span></div>
    </div>

js

  <script>
        const arr = ['Q', 'W', 'E', 'R', 'T', 'Y', 'U', 'I', 'O', 'P', 'A', 'S', 'D', 'F', 'G', 'H', 'J', 'K', 'L', 'Z', 'X', 'C', 'V', 'B', 'N', 'M']
        const ks = document.querySelector('.ks')
        const tz = document.querySelector('.tz')
        const span= document.querySelector('span')
        const ipt = document.querySelector('.itp')
        //  console.log(arr[ran])
         let i
        ks.addEventListener('click', function () {
           i= setInterval(function () {
                let ran = Math.floor(Math.random() * 26)
                ipt.innerHTML = arr[ran]

            }, 100)
           
        })
        tz.addEventListener('click',function(){
            clearInterval(i)
            span.innerHTML=ipt.innerHTML
        })
    </script>

效果：