Selecting previous siblings with CSS :has()

Question

I would like to target the previous sibling with css. The following article suggests it can be done. Article on how to select the previous sibling in css (https://tobiasahlin.com/blog/previous-sibling-css-has/)

However, it just won't work. Even on my code below.

.c-floating-label:has(+ .c-input) {
    color:blue;
}

h2:has(+ p) {
  color:blue;
}


.box:has(+ .circle) {
    background:blue;
}

<label for="fullName" class="c-floating-label">Full name</label>
<input type="text" name="fullName" id="fullName" placeholder="Full name" class="c-input">

<br/>

<h2>I am a heading</h2>
<p>I am a paragraph</p>

<br/>

<div class="box">Box</div>
<div class="circle">Circle</div>
<div class="box">Box</div>