Add persistent Unicode underline and double-underline marks that render on all devices without HTML.
A word-processor underline is formatting and disappears the moment you paste into a bio. A unicode underline is a combining mark, a low line character that attaches to the letter before it, and travels with the text. Two marks do the job: the combining low line U+0332 gives a single underline, and the combining double low line U+0333 gives a double. The tool appends one to every non-space character, so the line runs under each letter and breaks at spaces. The dotted, wavy and tilde underlines in the Effects family use other combining marks from the same block, and strikethrough uses the combining long stroke overlay U+0336 through the middle.
Because these are marks below the baseline, how they look depends on the font's line spacing. Instagram, X and Discord render the single and double underline cleanly. In tightly leaded text (some chat clients, some game fonts) the double line can touch the descenders of the line below. Underlined text is still the original letters, so it stays searchable and a screen reader reads it normally, which makes it the most accessible of the decorative effects. The one thing to know before using it: on iOS the underline can be drawn slightly heavier than on Android, so a double underline that looks right on one may look thick on the other.
Yes. It is a combining character attached to each letter, not formatting, so it pastes into bios, captions and comments and stays.
Yes. The letters are unchanged; the underline is an extra mark. Search matches the plain word, and screen readers read it normally.
iOS draws combining low lines slightly heavier than Android. The single underline looks similar on both; the double is where the difference shows.