What changed
Paste something into both boxes. When two lines look identical but are not, the reason appears underneath them.
Compared in this browser — nothing is uploaded and nothing is stored. Any diff tool can tell you that line 14 differs; this one tells you why two lines that look identical are not — a trailing space, Windows line endings, a non-breaking space from a CMS, a zero-width character copied out of a web page, Unicode normalisation, or a Cyrillic letter shaped like a Latin one.
The Diff / Text Compare tool needs JavaScript: the comparison runs in this browser, which is why nothing you paste has to be sent anywhere. With JavaScript switched off there is nothing to do the work.