Double Integral

∬

This character has no named entity, so the numeric form is the one to use.

\222C

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

\u222C

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

\u222C

The escape for a Python string literal.

%E2%88%AC

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

E2 88 AC

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