<!DOCTYPE html>
<html>
<body>
<input type="text" id="myInput" onblur="checkInput()">
<script>
function checkInput() {
var input = document.getElementById("myInput").value;
if (input < 10) {
document.getElementById("myInput").style.borderColor = "red";
} else {
document.getElementById("myInput").style.borderColor = "black";
}
}
</script>
</body>
</html>