# Reading Progress

> A thin line under the header that fills as you read the article, not the whole page. Cut at each section, it can show the minutes left, and runs on the scroll itself where the browser can.

Scroll · reading progress, progress bar, scroll, article, blog, time left, vanilla js · HTML, CSS, vanilla JS · no dependencies · updated 2026-11-06
Demo: https://kodza.dev/components/reading-progress/
Source on GitHub: https://github.com/kodza-dev/kodza/tree/main/components/reading-progress

Takes the whole screen (or reacts to the page scroll): give it a page, not a small box.

## Files

The code below, served as files: byte for byte what the Kodza site copies (the CSS includes its theme guard).

- [`kodza-reading-progress.css`](https://kodza.dev/registry/reading-progress/kodza-reading-progress.css) · 6.4 KB
- [`kodza-reading-progress.js`](https://kodza.dev/registry/reading-progress/kodza-reading-progress.js) · 11.3 KB
- [`kodza-reading-progress.html`](https://kodza.dev/registry/reading-progress/kodza-reading-progress.html) · 0.2 KB · the markup, to adapt and paste into your page

**Can you run shell commands? Download the CSS and the JS, don't retype them.** Retyping costs thousands of tokens and can drop or change lines of the CSS. Put the files where your stack expects them (Use it says where: change the `-o` paths):

```bash
curl -fsSL --create-dirs -o kodza/kodza-reading-progress.css https://kodza.dev/registry/reading-progress/kodza-reading-progress.css
curl -fsSL --create-dirs -o kodza/kodza-reading-progress.js https://kodza.dev/registry/reading-progress/kodza-reading-progress.js
```

Then load the files from your page as Use it shows, instead of pasting the content. The HTML is the part you adapt (text, links, modifiers): paste it from the Code section. Copy the CSS and the JS from the Code section only if you can't run commands, exactly as they are. (Windows PowerShell: `curl.exe`.)

## API

- Root: `[data-kodza-reading-progress]`
- Variables (set them on the root element itself: it declares its own defaults):
  - `--kz-reading-progress-offset: 0px` — the height of a fixed header above it
  - `--kz-reading-progress-height: 3px`
  - `--kz-reading-progress-color: var(--kz-color-ink, #0a0a0a)`
  - `--kz-reading-progress-track: color-mix(in srgb, var(--kz-reading-progress-color) 10%, transparent)`
  - `--kz-reading-progress-notch: 2px` — the cut at each heading; 0 for a plain line
  - `--kz-reading-progress-label-color: var(--kz-color-ink, #0a0a0a)`
  - `--kz-reading-progress-label-bg: var(--kz-color-paper, #ffffff)`
  - `--kz-reading-progress-label-border: color-mix(in srgb, var(--kz-reading-progress-label-color) 10%, transparent)`
  - `--kz-reading-progress-gutter: 16px` — the label's distance from the screen edge
  - `--kz-reading-progress-layer: 40`
- Attributes:
  - `data-done`
  - `data-headings`
  - `data-kodza-reading-progress`
  - `data-target`
  - `data-time`
- Lifecycle: `kodza:init` / `kodza:destroy` on `document`, with `{ detail: { root } }` to target a container.

## Dark mode

The values Kodza uses on a dark background (`#0a0a0a`). For a page that follows the system theme, add them after the component's CSS:

```css
@media (prefers-color-scheme: dark) {
  .kodza-reading-progress {
    --kz-reading-progress-color: #f2f2f0;
    --kz-reading-progress-track: rgba(242, 242, 240, 0.14);
    --kz-reading-progress-label-color: #f2f2f0;
    --kz-reading-progress-label-bg: #161616;
    --kz-reading-progress-label-border: rgba(255, 255, 255, 0.12);
  }
}
```

A site with its own theme switch puts the same rules under its dark selector instead (`[data-theme="dark"] .kodza-reading-progress { … }`); a single dark section, under that section's class.

## Use it

### Plain HTML page

- CSS: `kodza/kodza-reading-progress.css` next to the page (the Files commands as they are), linked in `<head>`: `<link rel="stylesheet" href="kodza/kodza-reading-progress.css">`. Without a shell, paste it in a `<style>` in `<head>` instead. It only touches `kodza-` classes.
- HTML: where the component goes. Paste it as many times as you need: instances never interfere.
- JS: `kodza/kodza-reading-progress.js`, loaded with `<script src="kodza/kodza-reading-progress.js" defer></script>` in `<head>`. Without a shell, paste it in a `<script>` before `</body>` (or in `<head>`: it waits for the page). One copy per page, whatever the number of instances.
- Restyle with the `--kz-…` variables, set on the component's root element itself (the root declares its own defaults, so a value on an ancestor is ignored), and with the modifiers / attributes listed under API.
- Font: it takes the page's own. A site with its own type sets it once for every Kodza component, on `:root`, never by editing the CSS (unset, nothing changes): `--kz-font-label`, the face of the small labels (meta, captions, counters, hints, footer links, button labels); `--kz-font-label-size`, their size, one for all of them (`11px`…); `--kz-font-label-weight`, their weight; `--kz-font-label-tracking`, their letter-spacing; `--kz-font-label-case`, their case (`uppercase`…; code and units keep theirs); `--kz-font-mono`, the monospace face (code, labels, counters). The Kodza look is Geist: `<link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Geist:wght@400;500;600;700&family=Geist+Mono:wght@400;500&display=swap">`.
- Corners: a site with its own shapes sets them once for every Kodza component, on `:root`, never by editing the CSS (unset, nothing changes): `--kz-radius-pill` sets the pills (`0` squares them).
- Motion: a site sets its pace once for every Kodza component, on `:root`, never by editing the CSS or the JS: `--kz-motion-scale` multiplies every duration (`0.5` twice as fast, `2` twice as slow, `0` no motion at all, loops stopped, like reduced motion). Unset, nothing changes.
- Content added later (AJAX navigation, a SPA route, a builder preview): start it with `document.dispatchEvent(new CustomEvent('kodza:init', { detail: { root: container } }))`; before removing it, the same call with `'kodza:destroy'` stops it and puts the original markup back.

### Claude artifact (claude.ai)

- Paste the CSS in a `<style>` and the JS in a `<script>` at the end, in the artifact itself: it loads stylesheets only from Google Fonts and scripts only from a few CDNs, so no `<link>` or `<script src>` to Kodza.
- Writing the artifact file with a shell? Insert the downloaded files (Files) with a command instead of retyping them: a line `KODZA-CSS` alone inside the `<style>`, `KODZA-JS` inside the `<script>`, then `sed -e '/^KODZA-CSS$/{r kodza/kodza-reading-progress.css' -e 'd' -e '}' -e '/^KODZA-JS$/{r kodza/kodza-reading-progress.js' -e 'd' -e '}' page.html > artifact.html`.
- Font: load Geist from Google Fonts (allowed): `<link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Geist:wght@400;500;600;700&family=Geist+Mono:wght@400;500&display=swap">`. A site with its own type sets it once for every Kodza component, on `:root`, never by editing the CSS (unset, nothing changes): `--kz-font-label`, the face of the small labels (meta, captions, counters, hints, footer links, button labels); `--kz-font-label-size`, their size, one for all of them (`11px`…); `--kz-font-label-weight`, their weight; `--kz-font-label-tracking`, their letter-spacing; `--kz-font-label-case`, their case (`uppercase`…; code and units keep theirs); `--kz-font-mono`, the monospace face (code, labels, counters).
- Corners: a site with its own shapes sets them once for every Kodza component, on `:root`, never by editing the CSS (unset, nothing changes): `--kz-radius-pill` sets the pills (`0` squares them).
- Motion: a site sets its pace once for every Kodza component, on `:root`, never by editing the CSS or the JS: `--kz-motion-scale` multiplies every duration (`0.5` twice as fast, `2` twice as slow, `0` no motion at all, loops stopped, like reduced motion). Unset, nothing changes.
- Dark mode: the viewer sets `data-theme="dark"` or `"light"` on `<html>`, or nothing when it follows the system. Write the Dark mode rules twice: inside `@media (prefers-color-scheme: dark)` with `:root:not([data-theme="light"])` in front of each selector, and with `:root[data-theme="dark"]` in front of each selector.

### WordPress

- HTML: a **Custom HTML** block (or a pattern / template part). WordPress keeps this markup as it is only for users allowed to post unfiltered HTML (Administrators; Super Admins on multisite): for anyone else it strips tags such as `<svg>`, `<input>` or `<dialog>`, so add it from the theme (a pattern or template part) instead.
- CSS and JS: the downloaded files go in the theme, under `wp-content/themes/<your-theme>/kodza/` (the child theme if there is one), enqueued from its functions.php (or a code-snippets plugin):
  ```php
  add_action('wp_enqueue_scripts', function () {
    $dir = get_stylesheet_directory() . '/kodza/';
    $uri = get_stylesheet_directory_uri() . '/kodza/';
    wp_enqueue_style('kodza-reading-progress', $uri . 'kodza-reading-progress.css', [], filemtime($dir . 'kodza-reading-progress.css'));
    wp_enqueue_script('kodza-reading-progress', $uri . 'kodza-reading-progress.js', [], filemtime($dir . 'kodza-reading-progress.js'), true);
  });
  ```
  Scoped to `kodza-` classes, the CSS never touches the theme. One copy of the JS, whatever the number of blocks.
- No access to the theme files? The CSS can also go, as it is, in **Appearance → Customize → Additional CSS** (classic theme) or **Styles → Additional CSS** in the Site Editor (block theme).
- Theme with AJAX page transitions: fire `kodza:init` on the new content after each swap (and `kodza:destroy` on the old one).
- Performance plugin (WP Rocket, LiteSpeed Cache, Perfmatters…) with **Delay JavaScript execution** turned on: add `kodza` to the scripts it must not delay. Otherwise the component waits for the visitor's first scroll, tap or mouse move, and that first click can be lost.
- Colours: it takes the colour of the text around it (or keeps its own monochrome): nothing to point at the theme.
- WordPress turns straight quotes and `--` into typographic ones in the text of a Custom HTML block (not inside `<code>` or `<pre>`).

#### With Elementor

- HTML: an **HTML** widget (Elementor shows it to administrators only). It keeps the code exactly as pasted, straight quotes included.
- CSS and JS: as above. Elementor's Site Settings → Theme Style styles every button, field, link and image of the page, components included; the theme guard at the end of the CSS keeps the component's buttons, fields and link colours as they are.
- The Elementor editor redraws each widget after the page has loaded: without the script below, the component stays still in the editor preview (the published page works). Add it once per site, in the footer like the component's script (`kodza/elementor-editor.js`, or a code-snippets plugin):

```js
// Kodza in the Elementor editor: the editor redraws each HTML widget after the page has loaded.
// Start Kodza in what it draws, stop it in what it replaces or removes. Once per site.
window.addEventListener('elementor/frontend/init', () => {
  if (!elementorFrontend.isEditMode()) return;
  class KodzaWidget extends elementorModules.frontend.handlers.Base {
    onInit() {
      super.onInit();
      this.kodza = [...this.$element[0].children];
      document.dispatchEvent(new CustomEvent('kodza:init', { detail: { root: this.$element[0] } }));
    }
    onDestroy() {
      super.onDestroy();
      this.kodza.forEach((root) => document.dispatchEvent(new CustomEvent('kodza:destroy', { detail: { root } })));
    }
  }
  elementorFrontend.elementsHandler.attachHandler('html', KodzaWidget);
});
```

- No child theme: everything can go in the HTML widget instead, `<style>` with the CSS, the HTML, then `<script>` with the JS. It works on the page and in the editor without the script above.

### Shopify

- Files: `assets/kodza-reading-progress.css` and `assets/kodza-reading-progress.js` (the Files commands with `-o assets/…`).
- In the section: `{{ 'kodza-reading-progress.css' | asset_url | stylesheet_tag }}` and `<script src="{{ 'kodza-reading-progress.js' | asset_url }}" defer></script>`, then the HTML.
- The theme editor re-renders sections: restart them with
  `document.addEventListener('shopify:section:load', (e) => document.dispatchEvent(new CustomEvent('kodza:init', { detail: { root: e.target } })));`
  and the same with `shopify:section:unload` → `kodza:destroy`.

### React / Next.js

- Files, side by side (e.g. in `components/kodza/`): `ReadingProgress.tsx` (below), the CSS block under Code saved as `reading-progress.css` and the JS block as `reading-progress.js`, both as they are. The HTML block is already inside the component: skip it.
- Use it: `import { ReadingProgress } from '@/components/kodza/ReadingProgress';` then `<ReadingProgress />`. Props: `time` (`data-time`), `vars` (the `--kz-…` variables listed under API, e.g. `vars={{ '--kz-reading-progress-offset': … }}`), `className`, `style` and the other props of its `<div>` (`onClick`, `aria-…`).
- Next.js App Router: it is a Client Component already (`'use client'`). Pages Router: Next.js takes global CSS only in `pages/_app`, so move the `import './reading-progress.css'` line there.
- Safe under StrictMode, client-side navigation and prop changes: when a prop changes, the `key` gives React a fresh element, `kodza:destroy` stops the script on the old one and `kodza:init` starts it on the new one.
- Built from your data (tabs, rows, options)? Add what the markup is made of to the `key`, as plain values (ids, text, numbers). What the visitor changes (the open tab, the sort, what they type) is the script's: follow it with `onClick` or `onInput` on the root, and change it from your code as a visitor would (click the tab; set the field's value, then dispatch a bubbling `input` event). Fields stay uncontrolled (`defaultValue`, `name`).

```tsx
'use client';
// Kodza — Reading Progress, for React. From https://kodza.dev/components/reading-progress/
// Kodza License: free to use, client work included. Not to resell or redistribute, ported or not: https://kodza.dev/docs/#print-license
// Next to this file: reading-progress.css and reading-progress.js, the component's CSS and JS from Kodza, as they are.
// Yours to edit: the markup is plain JSX below.
// The script changes this markup once React has rendered it (it adds layers, splits text, moves
// nodes), so React must not patch it afterwards. When a prop changes, the key gives React a fresh
// element: kodza:destroy stops the script on the old one, kodza:init starts it on the new one.
// data-kodza-manual keeps the script from starting it on its own: the effect below does, once
// React has hydrated the markup (on a page that streams in, the script can load first).
// Built from your data (tabs, rows, options)? Put what the markup is made of in the key, as plain
// values (ids, text, numbers): a change starts the script again on fresh markup. What the visitor
// changes (the open tab, the sort, what they type) is the script's: keep it out of the key, follow
// it with onClick or onInput on the root, and change it from your code as a visitor would (click
// the tab; set the field's value, then dispatch a bubbling input event).
import { useRef, useEffect, type ComponentPropsWithoutRef, type ComponentRef, type CSSProperties } from 'react';
import './reading-progress.css';

export type ReadingProgressProps = Omit<ComponentPropsWithoutRef<'div'>, "children" | "vars" | "time"> & {
  /** data-time on the root */
  time?: string;
  /** Settings, as CSS variables on the root */
  vars?: {
    /** the height of a fixed header above it — default: 0px */
    "--kz-reading-progress-offset"?: string | number;
    /** default: 3px */
    "--kz-reading-progress-height"?: string | number;
    /** default: var(--kz-color-ink, #0a0a0a) */
    "--kz-reading-progress-color"?: string | number;
    /** default: color-mix(in srgb, var(--kz-reading-progress-color) 10%, transparent) */
    "--kz-reading-progress-track"?: string | number;
    /** the cut at each heading; 0 for a plain line — default: 2px */
    "--kz-reading-progress-notch"?: string | number;
    /** default: var(--kz-color-ink, #0a0a0a) */
    "--kz-reading-progress-label-color"?: string | number;
    /** default: var(--kz-color-paper, #ffffff) */
    "--kz-reading-progress-label-bg"?: string | number;
    /** default: color-mix(in srgb, var(--kz-reading-progress-label-color) 10%, transparent) */
    "--kz-reading-progress-label-border"?: string | number;
    /** the label's distance from the screen edge — default: 16px */
    "--kz-reading-progress-gutter"?: string | number;
    /** default: 40 */
    "--kz-reading-progress-layer"?: string | number;
  };
  className?: string;
};

export function ReadingProgress({ time, vars, className, style, ...rest }: ReadingProgressProps) {
  const ref = useRef<ComponentRef<'div'>>(null);
  const classes = ["kodza-reading-progress", className].filter(Boolean).join(' ');
  // Everything the markup is made of: when it changes, React gets a fresh element (see the top)
  const key = JSON.stringify([classes, time, vars, style, rest]);

  // Starts the script on this element once React has rendered it, stops it when it goes
  useEffect(() => {
    const root = ref.current;
    let live = true;
    // @ts-ignore: a plain script, no types to find
    import('./reading-progress.js').then(() => {
      if (live) document.dispatchEvent(new CustomEvent('kodza:init', { detail: { root } }));
    });
    return () => {
      live = false;
      document.dispatchEvent(new CustomEvent('kodza:destroy', { detail: { root } }));
    };
  }, [key]);

  return (
    <div data-kodza-reading-progress="" key={key} {...rest} className={classes} data-kodza-manual data-time={time} style={{ ...vars, ...style } as CSSProperties} ref={ref}>
      <div className="kodza-reading-progress__bar" role="progressbar" aria-label="Reading progress">
        <span className="kodza-reading-progress__fill" />
      </div>
    </div>
  );
}
```

## Code

```html
<div class="kodza-reading-progress" data-kodza-reading-progress data-time>
  <div class="kodza-reading-progress__bar" role="progressbar" aria-label="Reading progress">
    <span class="kodza-reading-progress__fill"></span>
  </div>
</div>
```

```css
/* Kodza License: free to use, client work included. Not to resell or redistribute, ported or not: https://kodza.dev/docs/#print-license */
/* Kodza — Reading Progress
   A thin line fixed under the site header that measures the article alone: it slides in when the
   article reaches the header, fills as you read, leaves once the article has gone by, and is cut
   at each h2, so the fill crosses a notch when that heading passes under the header.
   Settings: the --kz-reading-progress-* variables below. On the root: data-target (the article,
   a CSS selector; default the closest article, main or .entry-content), data-headings (where the
   notches go, default h2), data-time (adds the time left: "{n} min left") and data-done (its last
   word, default "Done"). */
.kodza-reading-progress {
  --kz-reading-progress-offset: 0px; /* the height of a fixed header above it */
  --kz-reading-progress-height: 3px;
  --kz-reading-progress-color: var(--kz-color-ink, #0a0a0a);
  --kz-reading-progress-track: color-mix(in srgb, var(--kz-reading-progress-color) 10%, transparent);
  --kz-reading-progress-notch: 2px; /* the cut at each heading; 0 for a plain line */
  --kz-reading-progress-label-color: var(--kz-color-ink, #0a0a0a);
  --kz-reading-progress-label-bg: var(--kz-color-paper, #ffffff);
  --kz-reading-progress-label-border: color-mix(in srgb, var(--kz-reading-progress-label-color) 10%, transparent);
  --kz-reading-progress-gutter: 16px; /* the label's distance from the screen edge */
  --kz-reading-progress-layer: 40;

  display: block;
  height: 0;
  margin: 0;
}

.kodza-reading-progress__bar {
  position: fixed;
  top: var(--kz-reading-progress-offset);
  left: 0;
  right: 0;
  z-index: var(--kz-reading-progress-layer);
  display: block;
  height: var(--kz-reading-progress-height);
  margin: 0;
  overflow: hidden;
  background: var(--kz-reading-progress-track);
  pointer-events: none;
  /* hidden above its line until the article reaches the header */
  transform: translateY(-100%);
  visibility: hidden;
  transition:
    transform calc(0.32s * var(--kz-motion-scale, 1)) cubic-bezier(0.22, 1, 0.36, 1),
    visibility 0s linear calc(0.32s * var(--kz-motion-scale, 1));
}

.kodza-reading-progress.is-reading .kodza-reading-progress__bar {
  transform: none;
  visibility: visible;
  transition:
    transform calc(0.32s * var(--kz-motion-scale, 1)) cubic-bezier(0.22, 1, 0.36, 1),
    visibility 0s;
}

.kodza-reading-progress__fill {
  display: block;
  width: 100%;
  height: 100%;
  background: var(--kz-reading-progress-color);
  transform: scaleX(var(--kz-reading-progress-value, 0));
  transform-origin: left center;
}

.kodza-reading-progress:dir(rtl) .kodza-reading-progress__fill {
  transform-origin: right center;
}

/* Where the browser runs animations on the scroll itself, the fill follows the page there
   (no frame late); the script only gives it the article's start and end */
.kodza-reading-progress.is-timeline .kodza-reading-progress__fill {
  animation: kodza-reading-progress-fill linear both;
  animation-timeline: scroll(root block);
  animation-range: var(--kz-reading-progress-start, 0px) var(--kz-reading-progress-end, 100%);
}

@keyframes kodza-reading-progress-fill {
  from { transform: scaleX(0); }
  to { transform: scaleX(1); }
}

/* The time left, under the line, at the end of the screen */
.kodza-reading-progress__time {
  position: fixed;
  top: calc(var(--kz-reading-progress-offset) + var(--kz-reading-progress-height) + 10px);
  right: var(--kz-reading-progress-gutter);
  z-index: var(--kz-reading-progress-layer);
  display: inline-flex;
  align-items: center;
  box-sizing: border-box;
  height: 28px;
  margin: 0;
  padding: 0 12px;
  overflow: hidden;
  border: 1px solid var(--kz-reading-progress-label-border);
  border-radius: var(--kz-radius-pill, 999px);
  background: var(--kz-reading-progress-label-bg);
  color: var(--kz-reading-progress-label-color);
  font-family: var(--kz-font-label, var(--kz-font-mono, "Geist Mono", ui-monospace, "SF Mono", Menlo, monospace));
  font-size: var(--kz-font-label-size, 12px);
  font-weight: var(--kz-font-label-weight, 400);
  line-height: 1;
  letter-spacing: var(--kz-font-label-tracking, normal);
  text-transform: var(--kz-font-label-case, none);
  font-variant-numeric: tabular-nums;
  white-space: nowrap;
  pointer-events: none;
  /* unrolled from the line, like a tab pulled down */
  clip-path: inset(0 0 100% 0 round var(--kz-radius-pill, 999px));
  visibility: hidden;
  transition:
    clip-path calc(0.42s * var(--kz-motion-scale, 1)) cubic-bezier(0.65, 0, 0.35, 1),
    visibility 0s linear calc(0.42s * var(--kz-motion-scale, 1));
}

.kodza-reading-progress__time:dir(rtl) {
  right: auto;
  left: var(--kz-reading-progress-gutter);
}

.kodza-reading-progress.is-reading .kodza-reading-progress__time {
  clip-path: inset(0 0 0 0 round var(--kz-radius-pill, 999px));
  visibility: visible;
  transition:
    clip-path calc(0.42s * var(--kz-motion-scale, 1)) cubic-bezier(0.22, 1, 0.36, 1) calc(0.08s * var(--kz-motion-scale, 1)),
    visibility 0s;
}

/* Each word of the time sits in one cell: the old one leaves upward as the new one comes in */
.kodza-reading-progress__words {
  display: grid;
  overflow: hidden;
}

.kodza-reading-progress__word {
  grid-area: 1 / 1;
  /* At the start of the cell: while the cell narrows to a shorter word, the new one is whole */
  justify-self: start;
  display: block;
  padding: 2px 0;
}

/* On a phone the text runs the full width and the label would sit on its lines: the line alone */
@media (max-width: 559.98px) {
  .kodza-reading-progress__time {
    display: none;
  }
}

@media (prefers-reduced-motion: reduce) {
  .kodza-reading-progress__bar,
  .kodza-reading-progress__time,
  .kodza-reading-progress.is-reading .kodza-reading-progress__bar,
  .kodza-reading-progress.is-reading .kodza-reading-progress__time {
    transition: none !important;
  }
}

/* kodza-guard 5ffb0c19fc64 — generated by Kodza, do not edit.
   Theme guard: keeps a site's own CSS (WordPress themes, Bootstrap, CSS resets…) off this
   component. :not(#_) is always true and weighs as much as an id, so these lines win over
   theme rules made of classes and tags. Customise the component through its --kz-* variables. */

/* What the browser gives, on everything the component leaves to it */

.kodza-reading-progress:not(#_),
.kodza-reading-progress__bar:not(#_),
.kodza-reading-progress__fill:not(#_),
.kodza-reading-progress__words:not(#_),
.kodza-reading-progress__word:not(#_) {
  box-sizing: revert;
}
```

```js
// Kodza License: free to use, client work included. Not to resell or redistribute, ported or not: https://kodza.dev/docs/#print-license
// Kodza — Reading Progress
// Finds the article (data-target, or the closest article / main / .entry-content), measures it,
// and fills the line from the moment its top reaches the header to the moment its end reaches the
// bottom of the screen. The line comes in with the article and leaves once it has gone by. The
// notches (data-headings, default h2) are cut where the fill meets each heading as it passes under
// the header. data-time adds the minutes left, which roll when they change. The look is in the
// --kz-reading-progress-* variables (component.css), the reading speed just below.
(() => {
  const SELECTOR = '[data-kodza-reading-progress]';
  const WORDS_PER_MINUTE = 230; // reading speed for the time left
  const ROLL = 380; // ms, a word of the time left rolling to the next (times pace)
  const reduceMotion = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  // The site's pace, --kz-motion-scale (set on :root, read on the instance): every duration of the
  // script times it, and 0 moves nothing, like reduced motion
  const paceOf = (el) => {
    const pace = parseFloat(getComputedStyle(el).getPropertyValue('--kz-motion-scale'));
    return pace >= 0 ? pace : 1;
  };
  const clamp = (value, min, max) => Math.min(max, Math.max(min, value));
  const timeline = window.CSS?.supports?.('animation-timeline: scroll()') ?? false;

  // One instance, with its own state: two copies on a page never interfere. Everything it
  // starts is tied to `signal` (listeners get { signal }, observers and timers stop on its
  // 'abort' event), so kodza:destroy stops it all.
  function mount(root, signal) {
    const bar = root.querySelector('.kodza-reading-progress__bar');
    if (!bar) return;
    bar.setAttribute('aria-valuemin', '0');
    bar.setAttribute('aria-valuemax', '100');
    bar.setAttribute('aria-valuenow', '0');
    const selector = root.dataset.target || 'article, main, .entry-content';
    // The article around the line, or the closest one beside it
    let article = root.closest(selector);
    for (let scope = root.parentElement; !article && scope; scope = scope.parentElement) article = scope.querySelector(selector);
    if (!article) return;
    const pace = paceOf(root);
    const still = reduceMotion || !pace; // the still path: the line follows, nothing rolls
    const words = article.textContent.trim().split(/\s+/).length;

    // The time left: a pill under the line, each change rolling in from below
    let time = null;
    let cell = null;
    if (root.hasAttribute('data-time')) {
      time = document.createElement('span');
      time.className = 'kodza-reading-progress__time';
      time.setAttribute('aria-hidden', 'true');
      cell = document.createElement('span');
      cell.className = 'kodza-reading-progress__words';
      time.append(cell);
      root.append(time);
    }
    const format = root.dataset.time || '{n} min left';
    const done = root.dataset.done || 'Done';
    let said = '';
    const say = (text) => {
      if (text === said || !cell) return;
      const first = !said;
      said = text;
      const word = document.createElement('span');
      word.className = 'kodza-reading-progress__word';
      word.textContent = text;
      const old = [...cell.children].filter((el) => !el.classList.contains('is-leaving'));
      const from = cell.offsetWidth;
      cell.append(word);
      if (first || still || !time.offsetWidth) {
        old.forEach((el) => el.remove());
        return;
      }
      // the cell holds both while they cross: its width goes from the old word to the new one
      old.forEach((el) => el.classList.add('is-leaving'));
      const to = word.offsetWidth;
      const ms = ROLL * pace;
      const ease = 'cubic-bezier(0.22, 1, 0.36, 1)';
      cell.animate([{ width: `${from}px` }, { width: `${to}px` }], { duration: ms, easing: ease });
      word.animate([{ transform: 'translateY(105%)' }, { transform: 'none' }], { duration: ms, easing: ease });
      old.forEach((el) => {
        el.animate([{ transform: 'none' }, { transform: 'translateY(-105%)' }], { duration: ms, easing: ease, fill: 'forwards' })
          .finished.then(() => el.remove(), () => {});
      });
    };

    // Geometry, measured once and again when the page or the article changes size
    let start = 0;
    let end = 1;
    let offset = 0;
    const measure = () => {
      offset = parseFloat(getComputedStyle(bar).top) || 0;
      const box = article.getBoundingClientRect();
      const top = box.top + scrollY;
      start = top - offset;
      end = Math.max(start + 1, top + box.height - innerHeight);
      if (timeline) {
        root.style.setProperty('--kz-reading-progress-start', `${start.toFixed(1)}px`);
        root.style.setProperty('--kz-reading-progress-end', `${end.toFixed(1)}px`);
      }
      // a notch where the fill meets each heading as it passes under the header
      const cut = parseFloat(getComputedStyle(root).getPropertyValue('--kz-reading-progress-notch')) || 0;
      const rtl = getComputedStyle(root).direction === 'rtl';
      const at = [...article.querySelectorAll(root.dataset.headings || 'h2')]
        .map((h) => (h.getBoundingClientRect().top + scrollY - offset - start) / (end - start))
        .filter((p) => p > 0.01 && p < 0.99)
        .map((p) => (rtl ? 1 - p : p) * 100)
        .sort((a, b) => a - b);
      const mask = cut && at.length
        ? `linear-gradient(90deg, #000 0, ${at.map((p) => `#000 ${p.toFixed(3)}%, transparent ${p.toFixed(3)}%, transparent calc(${p.toFixed(3)}% + ${cut}px), #000 calc(${p.toFixed(3)}% + ${cut}px)`).join(', ')}, #000 100%)`
        : '';
      bar.style.maskImage = mask;
      bar.style.webkitMaskImage = mask;
      update();
    };

    let reading = false;
    let percent = -1;
    const update = () => {
      const box = article.getBoundingClientRect();
      // from the article reaching the header to the article having gone above it
      const now = box.top <= offset + 1 && box.bottom > offset;
      if (now !== reading) {
        reading = now;
        root.classList.toggle('is-reading', now);
      }
      const progress = clamp((scrollY - start) / (end - start), 0, 1);
      if (!timeline) root.style.setProperty('--kz-reading-progress-value', progress.toFixed(4));
      const value = Math.round(progress * 100);
      if (value !== percent) {
        percent = value;
        bar.setAttribute('aria-valuenow', value);
      }
      if (cell) {
        const left = Math.max(1, Math.round((words * (1 - progress)) / WORDS_PER_MINUTE));
        say(progress >= 0.995 || !left ? done : format.replace('{n}', left));
      }
    };

    let raf = 0;
    const queue = () => {
      if (!raf) raf = requestAnimationFrame(() => { raf = 0; update(); });
    };
    root.classList.toggle('is-timeline', timeline);
    measure();
    window.addEventListener('scroll', queue, { passive: true, signal });
    window.addEventListener('resize', measure, { signal });
    // the article grows as its pictures load, and moves when something above it does
    const resize = new ResizeObserver(() => measure());
    resize.observe(article);
    resize.observe(document.documentElement);
    signal.addEventListener('abort', () => {
      resize.disconnect();
      cancelAnimationFrame(raf);
    });
  }

  // Kodza lifecycle, the same in every component: `kodza:init` starts the instances inside
  // event.detail.root (the whole page by default), `kodza:destroy` stops them and puts their
  // markup back. For themes and apps that swap content without a page load. Each running root
  // keeps its markup as written under KODZA, shared by every Kodza script on the page: a Kodza
  // component inside another one is saved as written, not half-started, and stopped with it. Two
  // components on one root (a reveal on a stat card) both start, and stop together.
  const KODZA = Symbol.for('kodza');
  const RUNNING = Symbol.for(`kodza ${SELECTOR}`); // a root this component runs on, for every copy of this script
  const instances = new WeakMap();
  const find = (scope = document) => [scope, ...scope.querySelectorAll(SELECTOR)].filter((el) => el.matches?.(SELECTOR));
  const attrsOf = (el) => [...el.attributes].map((a) => [a.name, a.value]);
  const put = (el, saved) => {
    [...el.attributes].forEach((a) => el.removeAttribute(a.name));
    saved.attrs.forEach(([name, value]) => el.setAttribute(name, value));
    el.innerHTML = saved.html;
  };
  // The markup inside a root as written: Kodza components already running in it give back their own
  const written = (root) => {
    const all = [...root.querySelectorAll('*')];
    if (!all.some((el) => el[KODZA])) return root.innerHTML;
    const copy = root.cloneNode(true);
    const twins = copy.querySelectorAll('*');
    let last = null;
    all.forEach((el, i) => {
      if (!el[KODZA] || last?.contains(el)) return;
      last = el;
      put(twins[i], el[KODZA]);
    });
    return copy.innerHTML;
  };
  const init = (scope) => find(scope).forEach((root) => {
    // Already running, maybe from another copy of this script. Marked is-ready with no Kodza component
    // running on it, a root is a copy of a started one (a cloned slide): it is left as it is
    if (root[RUNNING] || (root.classList.contains('is-ready') && !root[KODZA])) return;
    // Rendered by React (data-kodza-manual): React starts it, with an init aimed at that markup once
    // it has hydrated it. Any earlier start would change markup React has not taken over yet
    const owner = root.closest('[data-kodza-manual]');
    if (owner && !(scope && owner.contains(scope))) return;
    const controller = new AbortController();
    // Another Kodza component already running on this root: its markup as written is kept, and the
    // two stop together
    const before = root[KODZA];
    const saved = { html: before ? before.html : written(root), attrs: before ? before.attrs : attrsOf(root), stop: () => destroy(root) };
    if (before) before.above = saved;
    instances.set(root, { controller, saved, before });
    root[KODZA] = saved;
    root[RUNNING] = true;
    mount(root, controller.signal);
    root.classList.add('is-ready');
  });
  const destroy = (scope) => find(scope).forEach((root) => {
    const instance = instances.get(root);
    if (!instance) return;
    instances.delete(root);
    instance.saved.above?.stop(); // a Kodza component started after it on the same root stops first
    // The Kodza components running inside it stop first: once its markup is put back, their own
    // script could not find them any more
    root.querySelectorAll('*').forEach((el) => el[KODZA]?.stop());
    instance.controller.abort();
    delete root[RUNNING];
    // Started on top of another one: that one stops next, and puts the markup back
    if (instance.before) {
      delete instance.before.above;
      root[KODZA] = instance.before;
      return instance.before.stop();
    }
    delete root[KODZA];
    put(root, instance.saved);
  });
  document.addEventListener('kodza:init', (event) => init(event.detail?.root));
  document.addEventListener('kodza:destroy', (event) => destroy(event.detail?.root));
  init();
  // Loaded in <head>, the script runs before the markup exists: once more when it is parsed
  if (document.readyState === 'loading') document.addEventListener('DOMContentLoaded', () => init(), { once: true });
})();
```

Free to use, from Kodza (https://kodza.dev).