Partial Sudoku Verifier

To practice my JavaScript for future employment, I’ve decided to take up the challenge of writing a javascript sudoku verifier. This code only verify’s one of the nine 3×3 blocks there are. I would like feedback on this portion of the code, so I can continue my development with more knowledge and more efficient code. Also, any feedback on my HTML/CSS is warmly welcome. I’m using a fullscreen chrome browser for development, so the #middle centers the box on my screen, but it might not on yours.

You will have to click on full page -> to view the CSS correctly.

function checkAnswer() {  	//reset each time button is clicked 	document.getElementById('correct').style.display = 'none'; 	document.getElementById('incorrect').style.display = 'none';  	//add each input into 2D array 	let first_row = document.getElementsByClassName('row1'); 	let second_row = document.getElementsByClassName('row2'); 	let third_row = document.getElementsByClassName('row3'); 	let sudoku = [ 		[first_row[0].value, first_row[1].value, first_row[2].value], 		[second_row[0].value, second_row[1].value, second_row[2].value], 		[third_row[0].value, third_row[1].value, third_row[2].value] 	]  	//check if each number is unique in the 2D array 	for (let i = 0; i < sudoku.length; i++) { 		for (let j = 0; j < sudoku[i].length; j++) { 			if(!isUnique(sudoku[i][j], sudoku)) { 				document.getElementById('incorrect').style.display = 'block'; 				return; 			} 		} 	} 	document.getElementById('correct').style.display = 'block'; }  function isUnique(num, arr) { 	let count = 0; 	//check entire array 	for(let i = 0; i < arr.length; i++) { 		for(let j = 0; j < arr[i].length; j++) { 			if(arr[i][j] == num) { 				count++; 			} 		} 	} 	return count == 1; }
body {  	background-color: pink;  } input {  	width: 50px;  	height: 50px;  	font-size: 20px;  	text-align: center;  } button {  	width: 100px;  	height: 50px;  	font-size: 15px;  	margin-left: 40px;  }  #middle {  	margin-left: 900px;  	margin-top: 300px;  }
<!DOCTYPE html> <html lang="en-US"> 	<head> 		<meta charset='UTF-8'> 		<title>Sudoku</title> 		<link rel="stylesheet" type="text/css" href="style.css"> 		<script src="script.js"></script> 	</head> 	<body> 		<div id="middle"> 			<table> 				<tr> 					<td><input type="text" class="row1" maxlength="1" value="3"></td> 					<td><input type="text" class="row1" maxlength="1"></td> 					<td><input type="text" class="row1" maxlength="1"></td> 				</tr> 				<tr> 					<td><input type="text" class="row2" maxlength="1"></td> 					<td><input type="text" class="row2" maxlength="1" value="2"></td> 					<td><input type="text" class="row2" maxlength="1"></td> 				</tr> 				<tr> 					<td><input type="text" class="row3" maxlength="1" value="7"></td> 					<td><input type="text" class="row3" maxlength="1"></td> 					<td><input type="text" class="row3" maxlength="1"></td> 				</tr> 			</table> 			<br> 			<button type="button" onclick="checkAnswer()">Check</button> 			<div id="incorrect" style="display: none;"> 				<h1>Incorrect Entry!</h1> 			</div> 			<div id="correct" style="display: none;"> 				<h1>Correct Entry!</h1> 			</div> 		</div> 	</body> </html>