Bootstrap 5.3.5 anchor tag with an image not clickable

I have the following base.html in my Quart template with bootstrap 3.5.3:

<div class="header">
  <a href="{{url_for('home.index')}}"><img src= "{{ url_for('static', filename='images/logo.png')}}" alt="logo" width="100" height="100"/></a>
  <h1 class="header-title">{{ title }}</h1>
</div>

css:

.header {
  height: 100px;
  vertical-align: middle;
}
.header img {
  float: left;
  width: 100px;
  height: 100%;
  background: #555;
}

.header h1 {
  position: relative;
  margin: 0 auto;
  height: 100px;
  height: 100%;
  text-align: center;
  color: rgb(0, 73, 101);
}
#header-title {
  position: absolute;
  text-align: center;
  display: inline;
  margin: 0 auto;
  height: 100px;
  height: 100%;
  color: rgb(0, 73, 101);
}

None of the UI elements is clickable on the browser. What do I miss?

The problem is likely that the heading element is overlapping the clickable anchor and any other elements inside the header. This is due to positioning and stacking order.

Try;

.header a {
  position: relative;
  z-index: 2;
}

.header h1 {
  position: relative;
  z-index: 1;
}

Setting position: relative; and a higher z-index on the <a> tag ensures it appears on top.