Scroll-Scrubbed Word Reveal

Make reading progress visible without replacing real text, breaking inline markup, or depending on a fixed line count.

Prepare the text

1. Keep one untouched accessible text source in the DOM.
2. Walk text nodes with TreeWalker; do not flatten the container with textContent or innerHTML.
3. Skip script, style, form controls, and elements marked with [data-no-split].
4. Replace only non-whitespace tokens with spans and preserve whitespace nodes exactly.
5. Mark generated spans aria-hidden="true" only when an equivalent unsplit accessible copy remains available.

Preferred structure:

<p class="reveal" data-reveal>
  Motion should <em>explain</em> the next state, not decorate it.
</p>