Smooth Scrolling Sticky ScrollSpy Navigation Using Pure CSS

How to make use of it:

1. Split your major content into a number of web page sections.

<section id="introduction">
  <h2>Introduction</h2>
  <p>…</p>
</section>
<section id="request-response">
  <h2>Request &amp; Response</h2>
  <p>…</p>
</section>
<section id="authentication">
  <h2>Authentication</h2>
  <p>…</p>
</section>
<section id="endpoints">
  <h2>Endpoints</h2>
  <section id="endpoints--root">
    <h3>Root</h3>
    <p>…</p>
  </section>
  <section id="endpoints--cities-overview">
    <h3>Cities Overview</h3>
    <p>…</p>
  </section>
  <section id="endpoints--city-detail">
    <h3>City Detail</h3>
    <p>…</p>
  </section>
  <section id="endpoints--city-config">
    <h3>City Config</h3>
    <p>…</p>
  </section>
  <section id="endpoints--city-spots-overview">
    <h3>City Spots Overview</h3>
    <p>…</p>
  </section>
  <section id="endpoints--city-spot-detail">
    <h3>City Spot Detail</h3>
    <p>…</p>
  </section>
  <section id="endpoints--city-icons-overview">
    <h3>City Icons Overview</h3>
    <p>…</p>
  </section>
  <section id="endpoints--city-icon-detail">
    <h3>City Icon Detail</h3>
    <p>…</p>
  </section>
</section>
<section id="links">
  <h2>Links</h2>
  <p>…</p>
</section>
<section id="expanders">
  <h2>Expanders</h2>
  <p>…</p>
</section>
<section id="filters">
  <h2>Filters</h2>
  <p>…</p>
</section>

2. Create the HTML for the sticky side navigation.

<nav class="section-nav">
  <ol>
    <li><a href="#introduction">Introduction</a></li>
    <li><a href="#request-response">Request &amp; Response</a></li>
    <li><a href="#authentication">Authentication</a></li>
    <li><a href="#endpoints">Endpoints</a>
      <ul>
        <li class=""><a href="#endpoints--root">Root</a></li>
        <li class=""><a href="#endpoints--cities-overview">Cities Overview</a></li>
        <li class=""><a href="#endpoints--city-detail">City Detail</a></li>
        <li class=""><a href="#endpoints--city-config">City Config</a></li>
        <li class=""><a href="#endpoints--city-spots-overview">City Spots Overview</a></li>
        <li class=""><a href="#endpoints--city-spot-detail">City Spot Detail</a></li>
        <li class=""><a href="#endpoints--city-icons-overview">City Icons Overview</a></li>
        <li class=""><a href="#endpoints--city-icon-detail">City Icon Detail</a></li>
      </ul>
    </li>
    <li class=""><a href="#links">Links</a></li>
    <li class=""><a href="#expanders">Expanders</a></li>
    <li class=""><a href="#filters">Filters</a></li>
  </ol>
</nav>

3. Style the side navigation.

.section-nav {
  width: 15em;
  position: fixed;
  top: 2rem;
  right: 2rem;  
  padding-left: 0;
  border-left: 1px solid #ddd;
  background-color: rgb(0 0 0 / 0);
}
.section-nav a {
  display: inline-block;
  text-decoration: none;
  line-height: 2rem;
  padding: 0 1rem;
  color: #202020;
}
.section-nav a:hover,
.section-nav a:focus {
  color: red;
  text-decoration: underline;
}

4. Enable the graceful scrolling impact.

html {
  scroll-behavior: smooth;
}

5. The major CSS to allow the scroll spy functionality.

:root {
  --linkheight: 2rem;
}
section {
  padding-bottom: 20rem;
}
section:nth-of-type(even) {
  padding-bottom: 70rem;
}
section:nth-of-type(1) {
  background:
    right 2rem top 2rem / 15em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(2) {
  background:
    right 2rem top calc(2 * var(--linkheight)) / 15em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(3) {
  background: right 2rem top calc(3 * var(--linkheight)) / 15em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(4) {
  padding-bottom: 0;
  background: right 2rem top calc(4 * var(--linkheight)) / 15em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(4) section:nth-of-type(1) {
  background: right 2rem top calc(5 * var(--linkheight)) / 14em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(4) section:nth-of-type(2) {
  background: right 2rem top calc(6 * var(--linkheight)) / 14em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(4) section:nth-of-type(3) {
  background: right 2rem top calc(7 * var(--linkheight)) / 14em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(4) section:nth-of-type(4) {
  background: right 2rem top calc(8 * var(--linkheight)) / 14em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(4) section:nth-of-type(5) {
  background: right 2rem top calc(9 * var(--linkheight)) / 14em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(4) section:nth-of-type(6) {
  background: right 2rem top calc(10 * var(--linkheight)) / 14em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(4) section:nth-of-type(7) {
  background: right 2rem top calc(11 * var(--linkheight)) / 14em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(4) section:nth-of-type(8) {
  background: right 2rem top calc(12 * var(--linkheight)) / 14em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(5) {
  background: right 2rem top calc(13 * var(--linkheight)) / 15em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(6) {
  background: right 2rem top calc(14 * var(--linkheight)) / 15em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(7) {
  background: right 2rem top calc(15 * var(--linkheight)) / 15em var(--linkheight) no-repeat linear-gradient(#ccc, #ccc);
}
section:nth-of-type(n),
section:nth-of-type(4) section:nth-of-type(n){
  background-attachment: fixed;
}

Sticky Scrollspy Navigation With Smooth Scroll Plugin/Github, sticky navbar css only, scrollspy plugins

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