javascript - 验证码不会自动显示并在单击刷新时闪烁。不知道该怎么做

Question

所以这是我用于创建验证码场景的代码，我遇到的问题是当我加载注册表单时验证码不会自动显示，当我点击刷新按钮时验证码确实可见但不到一秒钟。css 与它有什么关系吗？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>