Base64 Encode / Decode converts text or a file to Base64 and back. Text is handled as UTF-8, so any language and emoji round-trip, and there is a URL-safe option that uses dash and underscore and drops the padding.
Files are read with the browser File API and encoded in the page. Nothing is uploaded and no copy is kept on a server.
No. Encoding and decoding run in JavaScript in the open tab.
Yes. Text is encoded as UTF-8, so any script and emoji survive a round trip.
The text view says so. Use file mode to save the decoded bytes instead.
No. Whitespace and missing = padding are tolerated, and URL-safe input is accepted.