- Make your image
img {width: 100%; height: 100%;}
- Make your bigbox class display: flex .
Code below -
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
body {margin:0;
padding:0;}
img {
width: 100%;
height: 100%;
}
a { text-decoration:none; }
.box50 {width:50%;
height:auto;
float:left;}
#bigbox {width:100%;
height:auto;
display:block;
overflow:hidden;
display: flex;}
.box50_tr {display: inline-block;
width: 100%;}
.box50_td {
width: 100%;
display: block;
}
</style>
</head>
<body>
<div id="bigbox">
<div class="box50">
<img src="https://www.rakuten.ne.jp/gold/sappun/promotion/20201231/test_07.jpg">
</div>
<div class="box50">
<table cellpadding="0" cellspacing="0" style="display:inline-block;">
<tr class="box50_tr">
<td class="box50_td"><img src="https://www.rakuten.ne.jp/gold/sappun/promotion/20201231/test_08.jpg"></td>
</tr>
<tr class="box50_tr">
<td class="box50_td"><img src="https://www.rakuten.ne.jp/gold/sappun/promotion/20201231/test_09.jpg"></td>
</tr>
</table>
</div>
</div>
</body>
</html>
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