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⇑

A named entity — readable, and safe in any HTML document without declaring a charset.

⇑

The numeric form works everywhere a named one does, and exists for every character.

\21D1

For a content property in a ::before or ::after rule. No semicolon, and mind the trailing space.

\u21D1

Valid in a JavaScript or JSON string literal, so it survives files and pipelines that are not UTF-8 clean.

\u21D1

The escape for a Python string literal.

%E2%87%91

Percent-encoded UTF-8, for a query string or a path segment.

E2 87 91

3 bytes on the wire. Seeing these as separate garbled characters means something decoded them as single-byte text.