Tofu

Tofu is the blank rectangle you get when a font is asked to draw a character it has no shape for. The nickname is the resemblance to a block of bean curd, and it stuck hard enough to name a typeface project: Noto is short for “no more tofu”, and its stated goal is a glyph for every character Unicode encodes, in every script, so that no reader anywhere meets a row of boxes where their language should be.

Not every box means the same thing. The one the font draws itself is .notdef, glyph zero, the shape every font file reserves for “I was asked for this and I have nothing”; most faces draw it as a plain rectangle, and a few draw nothing at all, which is worse, because a character that fails silently is invisible until somebody counts. A box with four tiny hexadecimal digits inside it is a last resort face telling you the codepoint it could not set, which makes it the most useful box of the three. And U+FFFD, the replacement character, is a different failure entirely: it is not a missing glyph but a run of bytes that was not valid text, put there by whatever decoded the file, so it points at an encoding bug rather than at a font.

Tofu is what the end of a font stack looks like when it runs out. The browser walks the families you listed, then the system’s own fallback fonts, and draws a box only when nothing on the machine covers the character. Subsetting a web font to the Latin range is the most reliable way to manufacture one: the page looks immaculate until a user types their own name. Emoji are the other common source, since a codepoint asking for emoji presentation needs a colour emoji font present at all, and an icon font shows the same rectangle when its file fails to load, except that there the box stands in for a control nobody can now name.

What shipping tofu tells a reader is that their name, their language, or their script was not in the room when the product was built. The fixes are unglamorous: keep a broad fallback at the end of every stack, subset by the characters your users actually enter rather than by the ones in your marketing copy, and test with text you did not write yourself. A box in a screenshot is a bug report, not a rendering quirk.