验证码不会自动显示并在单击刷新时闪烁不知道如何去做

如何解决验证码不会自动显示并在单击刷新时闪烁不知道如何去做

所以这是我用来创建验证码场景的代码我遇到的问题是验证码在我加载注册表时不会自动显示，当我点击刷新按钮验证码确实可见但不到一个第二。 css 必须用它做些什么吗？建议plz是的！

    <label>Enter Captcha : </label>
    <input type="text" id="concaptcha">
    <button class="confcaptcha" onclick="validCap()"><b>Confirm</b></button>

    <script type="text/javascript">
    function capVar(){
        var apfa=['A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z','1','2','3','4','5','6','7','8','9','0','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'];
        var o = apfa[Math.floor(Math.random()*62)];
        var t = apfa[Math.floor(Math.random()*62)];
        var th = apfa[Math.floor(Math.random()*62)];
        var f = apfa[Math.floor(Math.random()*62)];
        var fv = apfa[Math.floor(Math.random()*62)];
        var s = apfa[Math.floor(Math.random()*62)];

        var total = o + t + th + f + fv + s ;

        document.getElementById("captcha").value = total;           
    }

    function validCap(){
        
        var c1 = document.getElementById("captcha").value;
        var c2 = document.getElementById("concaptcha").value;
        if (c1==c2){
            alert("Sucess!");
            return true;
        }
        else{
            alert("Invalid Captcha! Try again.  ")
            return false;
        }
    }
    </script>
    </div>