Uppercase converter

Uppercasing can change the length of a string, which is the practical trap. German ß becomes SS and the fi ligature becomes FI: one character in, two out. In a document that is invisible; in a fixed-width database column, a field with a character limit or a layout budgeted to the character, an uppercased value can suddenly overflow. It does not round-trip either: lowercasing SS gives ss, and nothing brings the ß back.

Which raises the question of whether the characters should change at all. If the capitals are for looks. A heading, a button, a label; the CSS text-transform: uppercase is the better answer, because the stored text stays as written, so copying it, searching it and reading it aloud with a screen reader all still work on the real words. Convert the characters only when the value itself has to be capitals. And there is a reason not to set much text this way: readers use the outline of a word as a cue, and capitals give every word the same rectangle, which measurably slows reading. Capitals are for short labels and emphasis, not for paragraphs.