Scrolling is something we all know and do on the web to the extent that it’s an expectation or perhaps even a habit, like brushing our teeth. That’s probably why we don’t put too much thought into designing the scrolling experience — it’s a well-known basic function. In fact, the popular “there is no fold” saying comes from the idea that people know how to scroll and there is no arbitrary line that people don’t go under.

Scroll-based features tend to involve some bespoke concoction of CSS and JavaScript. That’s because there simply aren’t that many native features available to do it. But what if we could accomplish something that only uses CSS? 

Take this ingenious horizontal scrollbar with CSS, for instance. I want to do something similar, but to indicate scrolled sections rather than capture continuous scrolling. In other words, rather than increasing the length of the indicator during scroll, I only want to increase the length when a certain section of the page has been reached.

Like this:

Here’s my plan: Each section carries an indicator that’s undetectable until it reaches the top of the screen. That’s where it becomes visible by changing color and sticks to the top of the viewport.

The exact opposite should happen in reverse: the indicator will follow along when scrolling back up the screen, camouflaging itself back to being undetected to the naked eye.

There are two key parts to this. The first is for the indicator to change color when it’s near the top of the screen. The second is for the indicator to stay put at the top of the screen and come down only when its section is scrolled down to.

The second one is easy to do: we use position: sticky; on our elements. When a page is scrolled, a sticky element sticks to a given position on the screen within its parent container.

That brings us to changing colors. Since the background of an HTML document is white by default, I’m keeping white as the base color for the demo. This means the indicator should look white when it’s over the base color and turn to some other color when it’s over the indicator bar at the top of the screen.

This is where CSS blend modes come into play. They give us so many options to create a variety of color amalgams. I’m going to go with the overlay value. This one is quite dynamic in nature. I won’t explain the blend in depth (because the CSS-Tricks Alamanac already does a good job of that) but taking this demo into account, I’ll say this: when the background color is white the resulting foreground color is white; and when the background is some other color, the resulting color is darker or lighter, depending on the color it’s mixed with.

The indicator stops in the demo are black. But, because of the blend, we see them as white because they are on a white background. And when they are over the indicator container element, which is a lovely shade of violet, we see a dark violet indicator stop, because we’re mixing the indicator stop’s black with the indicator container’s violet.

Starting with the HTML:

<div id="passageWrapper">
  <strong>Sections Scrolled ↴</strong>
  <!-- Indicator container -->
  <div id="passage"></div>
</div>


<!-- Indicator stop -->
<div class=passageStops></div>


<!-- First Section -->
<div class="sections">
  <!-- Content -->
</div>


<!-- Another indicator stop -->
<div class="passageStops"></div>


<!-- Second Section -->
<div class="sections">
  <!-- Content -->
</div>


<!-- Another indicator stop -->
<div class="passageStops"></div>


<!-- Third Section -->
<div class="sections">
  <!-- Content -->
</div>

Pretty straightforward, right? There’s a sticky container at the very top that holds the indicators when they reach the top.  From there, we have three sections of content, each one topped with an indicator that will stick to the top with the indicator and blend with it.

Here’s the CSS:

.passageStops {
  background-color: black; /* Each indicator stop is black */
  mix-blend-mode: overlay; /* This makes it appear white on a white background */
  width: 33.3%; /* Three sections total, so each section is one-third */
  top: calc(1em + 3px);
}


#passage, 
.passageStops{
  height: 10px;
}


#passageWrapper,
.passageStops {
  position: sticky; /* The container and stops should stick to the top */
  z-index: 1; /* Make sure the indicator and stops stay at the forefront */
}


#passage {
  background: violet; /* Will blend with black to make a darker violet indicator */
  margin: 0 0 20px 0;
}


#passageWrapper{
  background-color: white; /* Make sure we're working with white to hide indicator stops */
  height: 40px;
  top: 0px;
}


/* Each stop will shift one-third the width of the indicator container to cover the whole thing when the last section is reached. */
.passageStops:nth-child(4){ margin-left: 33.3%; }
.passageStops:nth-child(6){ margin-left: 66.6%; }


/* More styling, blah blah. */

The indicators (.passageStops) are black. But the overlay blend mode makes them appear white when it blends with the white background under it. Since there are three sections, each indicator is of one-third width.

The indicators have position: sticky; with a top distance value. This means the indicators will stick once they reach the calculated position from the top of the screen. When that happens, the black indicators that appeared white blend with the violet indicator container, which makes them appear to be a dark violet, representing the new scroll position on the page.

The reverse is also true. When an indicator loses its sticky position, it will move from the violet background of the indicator bar to the white background of the page, hiding it once again… like it was never there!

Here’s the demo again:

That’s it. You can perhaps further experiment with this by having a non-white background with another blend mode, or a gradient for the indicator bar or stops.