<div style="width:800px; height:600px; background-image: url('https://lorempixel.com/800/800/city'); background-size:cover; padding:30px;">
    <div class="bg-white-opacity">
      <h1>梅問題教學網</h1>
    </div>
</div>
.bg-white-opacity {
  background-color: rgba(255, 255, 255, 0.85);
  padding:40px; 
  margin:0 auto;
  width:400px;  
  height:400px;
  mix-blend-mode: screen;
}
h1{
  color:#000;
  font-size:120px;
}

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