function name will be validated not Validate.Html Uppercase function name Validate/Lower case in validate JS
<!-- begin snippet: js hide: false console: true babel: false -->
<!-- language: lang-js -->
function validate(){
if (document.getElementById("textbox_1").value==''){
document.getElementById("textbox_1").style.backgroundColor='red';
}
}
<!-- language: lang-html -->
<!DOCTYPE HTML>
<HTML>
<head>
</head>
<body>
NAME:<INPUT TYPE="Text" SIZE="15" VALUE="" ID="textbox_1">
<INPUT TYPE="Button" VALUE="SEND" onClick="validate()">
</body>
</html>
<!-- end snippet -->
You have written he function name wrong
<!-- begin snippet: js hide: false console: true babel: false -->
<!-- language: lang-js -->
function validate(){
if (document.getElementById("textbox_1").value=='')
document.getElementById("textbox_1").style.backgroundColor='red';
}
<!-- language: lang-html -->
NAME:<input type="Text" size="15" value="" id="textbox_1">
<input type="Button" value="SEND" onClick="validate()">
<!-- end snippet -->
