Character Conversion Formats
Every format starts with a code point
A character such as the heart suit U+2665 can be copied as the literal character, an HTML entity, a CSS escape, or a JavaScript escape. The code point stays the same.
The conversion tools on this site compute all forms from the same code point, so the values are consistent by construction.
Common escape syntax
HTML uses ♥ or ♥. CSS uses \2665 or \002665. JavaScript and Python use \u2665 inside the BMP and \u{1F600} or \U0001F600 for astral characters.
URL encoding percent-encodes the UTF-8 bytes, so the heart becomes %E2%99%A5.
Choose by destination
Use the literal character in documents, HTML entities in web markup, CSS escapes in stylesheets, and JavaScript or Python escapes in source code.
Copying the wrong syntax usually explains why a symbol appears as text instead of its intended glyph.