Why a ruler
Losing your line is a tax on every paragraph.
A dyslexic reader doesn't read less — they re-read more. The eyes leave the text all day: to think, to take a note, to check a diagram. Every return means hunting through a block of near-identical lines for the one that was yours. We have not measured that in a lab and we are not going to pretend we have. Ask any reader who does it, though, and they will tell you the hunt is the part that turns one page of homework into an evening.
Linelighter shortens the hunt to one glance — the band is already on the line you left. A tinted band sits under your cursor, so your place is marked by the thing your hand already moves. Rest the pointer in the margin and read; when your eyes come back, the band is exactly where you left it.
It stays out of your way, too. The ruler injects only when you call it — a toolbar click or Alt+R — and runs on no page you didn't ask it to touch. There are no host permissions in the free tier. Settings live in your Chrome profile and roam with it, so a school Chromebook remembers you by lunchtime. No account. Nothing about your reading leaves the browser.