Scroll Progress Timeline

Use one progress line to connect ordered information. The sequence must remain complete, readable, and navigable before animation is added.

Model the steps

Keep the content data-driven:

const steps = [
  { id: "brief", number: "01", title: "Set the direction", body: "..." },
  { id: "build", number: "02", title: "Make the system", body: "..." },
  { id: "ship", number: "03", title: "Release and learn", body: "..." }
];

Render it as an ordered list with real headings. The line, dots, media, and active state enhance that structure; they do not replace it.

Build the line

1. Render a quiet base line behind every point.
2. Place one progress line on top with transform-origin: top for vertical or left for horizontal.
3. Measure the first and last point centers, not arbitrary section edges.
4. Normalize scroll position between those centers.
5. Apply scaleY(progress) or scaleX(progress) so updates stay on the compositor.
6. Mark a step active when the progress head crosses its center.

const progress = Math.min(1, Math.max(0,
  (viewportAnchor - lineStart) / (lineEnd - lineStart)
));
line.style.transform = `scaleY(${progress})`;

Schedule DOM writes in one animation frame. Recalculate geometry after font and image loading, resize, orientation changes, and content mutation.

Choose the layout

• Use a centered alternating timeline only when both sides have enough width and similar content weight.
• Use a left rail for long copy, compact steps, or mixed card heights.
• Use a horizontal line for short sequences with concise labels and explicit keyboard-safe overflow.
• Use pinned full-screen chapters only when each step carries a distinct visual state. Keep the pin finite and release before the next section.
• Collapse to a simple left rail on small screens. Do not preserve alternation at the expense of reading order.

Animate step state

Use small opacity, translate, scale, blur, color, or media transitions. Keep every step readable while inactive. Expose active index with aria-current="step" only when that state is meaningful and current; do not announce every scroll update with a live region.

Use IntersectionObserver for simple active-state entry. Use a normalized scroll measurement or GSAP ScrollTrigger when the line must fill continuously or coordinate pinned media.

Handle navigation

• Make step links real anchors when users can jump within the process.
• Add scroll-margin for sticky headers.
• Preserve focus and do not move it during passive scrolling.
• Keep URLs and browser history stable unless the user explicitly selects a step.
• If steps are interactive, use buttons or links with visible focus; never make a decorative dot the only control.

Reduce motion

Under prefers-reduced-motion: reduce, show the complete line or discrete reached states without scrubbed interpolation, blur, pinning, or large transforms. Keep ordinary document flow and all step content.

Verify

Test variable step counts, uneven card heights, missing media, long translations, 390/768/1024/1440 widths, 200% zoom, fast forward and reverse scrolling, direct anchor navigation, keyboard order, reduced motion, late font/image layout, route cleanup, and console errors. The active step and line head must agree at every boundary.

Use demo/index.html as the working reference and demo/PROMPT.md to recreate or remix it. Keep REFERENCES.md as the links-only implementation source list.