我在给表格的每个单元格加边框时遇到了麻烦，它只显示出不带边框的表格。
我想让它显示带有边框的表格。
我尝试使用 w3schools 中的 css 样式来为表格添加边框，但好像并没有起作用，我还尝试了在样式中加入 !important 标记，但依然无效...
html：
<!DOCTYPE html>
<?php
$database = "seriousdiseases";
$database = new mysqli('localhost', 'root', '', $database) or die ("Unable to connect to the
DB");
?>
<html>
<head>
<title>Ribose-5-phosphate isomerase deficiency</title>
<link rel="stylesheet" type="text/css" href="style.css"/>
</head>
<body>
<div id="container2">
<div id="header">
<h1>Ribose-5-phosphate isomerase deficiency</h1>
</div>
<div id="content">
<div id="nav">
<h3>Navigation</h3>
<ul>
<li><a href="index.html">Home</a></li>
<li><a href="aboutUs.html">About Me</a></li>
<li><a href="video.html">Video</a></li>
<li><a href="seriousDiseases.html">Serious Diseases</a></li>
<li><a href="updateDiseases.php">Update Diseases</a></li>
<img src="F1large.jpg" alt="Navigation Picture." style="height:500px;">
</ul>
</div>
<div id="main">
<table>
<tr>
<th>Id</th>
<th>Disease</th>
<th>Symptoms</th>
</tr>
<?php
$result = mysqli_query($database, "select * from feature") or die
("Unable to connect to the DB");
while ($row = mysqli_fetch_array($result)){
print("<tr><td>" . $row[0] . "</td><td>" . $row[1] . "</td><td>"
. $row[2] . "</td></tr>");
}
$database->close();
?>
</table>
</div>
</div>
</div>
<div id="footer">
Copyright © 2020 CLINIC SCOPE
</div>
</body>
</html>
CSS：
html, body {
height: 100%;
}
body {
background-color: #EEE;
font-family: Helvetica, Arial, sans-serif;
background-image: url (logo.png);
margin-left: 0px;
margin-right: 0px;
margin-top: 0px;
display: flex;
flex-direction: column;
}
table, th, td{
border: 1px solid black !important;
}
h1, h2, h3 {
margin: 0;
}
a {
text-decoration: none;
color: blue;
}
#container {
background-color: white;
margin-left: 0px;
margin-right: 0px;
position: relative;
min-height: 97.7vh;
}
#container2 {
background-color: white;
margin-left: 0px;
margin-right: 0px;
position: relative;
}
#header {
background-color: blue;
color: white;
text-align: center;
padding: 10px;
}
#content {
padding:10px;
margin-right:0px;
}
#nav ul {
list-style-type: none;
}
#nav .selected {
font-family: bold;
}
#nav {
width: 18%;
float: left;
}
#main {
width: 60%;
float: right;
}
#footer {
color: white;
background-color: black;
width: 100%;
text-align: center;
position: relative;
bottom: 0;
}