What these watermarks are
Subtitles downloaded online often get extra entries at the start or end (sometimes in the middle too): URLs, email addresses, fansub group names, `encoded by` / `translated by` credits, site names like opensubtitles, and lines like “follow our WeChat account” or “for study purposes only”. They are normal subtitle entries — with a timeline, taking up a line — so players cannot remove them; they have to be detected by content.
Where to do it
Open the Subtitle Cleaner and drop in your subtitle; the “Watermarks & junk entries” group already has “Remove watermarks and ads”, “Remove duplicate entries” and “Remove empty entries” checked by default, and cleaning starts automatically. Detection works on two levels: hard signals like URLs, emails, fansub groups and site names delete the whole entry on a hit at any length; soft signals like @handles, “encoded” or “proofread” credits are only judged in short entries of no more than 40 characters, to avoid clipping long dialogue. Bilingual entries first have watermark lines stripped line by line while dialogue lines are kept; the entry is deleted whole only if watermark traces remain after stripping.
Review and export
In the change list, every deleted entry carries a “watermark & ad” tag — skim it to confirm nothing was removed by mistake; if one looks wrong, click “Restore” to put it back. Once confirmed, download: the file name gets a `.clean` marker automatically. Cleaning runs locally in your browser; your subtitles are never sent anywhere.