A cursive font generator doesn't create or install a font — it maps each letter you type to a different, pre-existing Unicode character that happens to look script-like. Understanding that mapping explains both why the trick works everywhere and why it has a few hard limits worth knowing before you rely on it.
The Unicode block behind cursive text
This site's cursive style pulls from the Mathematical Script letters inside Unicode's Mathematical Alphanumeric Symbols block (U+1D400–U+1D7FF) — a range originally added so mathematicians could write script-style variables (like 𝒜 for a set) without relying on a specific font. Text generators repurpose that same range for decorative cursive-look text, since visually it reads as handwriting-style script even though its official Unicode purpose is mathematical notation.
Why it isn't a 'real' font
A real font is a file that defines how existing characters are drawn. What a cursive font generator does instead is character substitution: your letter A becomes the character 𝒜, a completely different code point that every device already has a built-in appearance for, drawn in whatever font that device happens to use. That's why cursive text looks slightly different from app to app — you're seeing one Unicode character rendered in different system fonts, not one custom font rendered consistently.
The practical limits this creates
Because it relies on a specific, fairly obscure Unicode block, support isn't universal — a few very old devices or minimal fonts don't include glyphs for U+1D400 and beyond, which is when you'll see boxes instead of letters. Screen readers also handle this range inconsistently; some announce the letters normally, others skip or garble them, which is one of the accessibility trade-offs covered honestly in Unicode Explained. And because the underlying characters are mathematical letters, not stylistic markup, search engines and some apps may not treat cursive text as equivalent to the plain word — worth keeping plain-text alternatives for anything that needs to be findable.