Flawless might be a bit of a stretch. You're looking for a site with zero friction—no email, no password, no account linking. Those exist, but the selection is usually older public domain works or very niche fan translations. Gutenberg is the classic for that; you click, you read, done. But for anything remotely recent, 'free' and 'no signup' often means the site's revenue comes from invasive ads or worse.
I tried a few that promised this, and the experience was... not flawless. Pages reloading with pop-ups, broken chapter navigation. Sometimes the trade-off isn't worth it. If a story seems too current to be freely available with no strings, it probably is.
Your safest bet for a clean, legal read with no sign-up is still your local library's digital portal, though that does require a library card, so it's not what you're asking for here. For true zero-barrier, you're in public domain territory.
Most of the time, it's a 'pick two' situation: free, legal, and easy rarely all align.
That said, some smaller web novel platforms that run on ad revenue alone will let you read without an account. Just be ready for a less polished reading experience.