Untitled

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        form#formLogar{
            display: none;
        }
        form#formCadastrar{
            display: none;
        }
    </style>
</head>
<body>

    <button id="log" onclick="logar();">Logar</button>
    <button id="cad" onclick="cadastrar();">Cadastrar</button>

    <form action="" id="formLogar" method="post">
        Email: <input type="email" name="emailLog"><br>
        Senha: <input type="password" name="senhaLog"><br><br>
        <input type="submit" value="Logar" onclick="logar();">
    </form>

    <form action="" id="formCadastrar" method="post">
        Email: <input type="email" name="emailCad"><br>
        Senha: <input type="password" name="senhaCad"><br>
        Confirma senha: <input type="password" name="confSenha"><br><br>
        <input type="submit" value="Cadastrar" onclick="cadastrar();">
    </form>

    <script>
        function logar(){
            var log = document.getElementById("formLogar").style.display = "block";
            var formCadastrar = document.getElementById("formCadastrar");
            formCadastrar.style.display = "none";
        }

        function cadastrar(){
            var cad = document.getElementById("formCadastrar").style.display = "block";
            var formLogar = document.getElementById("formLogar");
            formLogar.style.display = "none";
        }
    </script>

</body>
</html>