• HTML
• CSS
• JS

HTML

<div class="alfabeto">ABCDEFGHIJKLMNÑOPQRSTUVWXYZ</div>
<div id="letra"></div>

CSS

body{
    font-family:arial;
    background:#ec7063;
}
#letra{
    font-size:300px;
    color:#d6dbdf;
    text-align:center;
}
.alfabeto{
    text-align:center;
    color:white;
}

JS

var i  = 0; // contador
var letra = document.getElementById("letra");
var abc   = "ABCDEFGHIJKLMNÑOPQRSTUVWXYZ";

window.setInterval(function(){

        if(i == abc.length){ i=0; }
        
        letra.innerHTML = abc[i]; // imprimir
        i++;
    
},750);