caret-shape allow web developers to override the caret visibility. However, while there is IME composition, browsers should (or may need to) ignore the style. Basically, IME composition string is designed for vertical bar style caret or no caret during composition. Additionally, like some Korean IME on Windows, IME may be designed to use box style caret to select entire the compositing Hangul character. Therefore, the spec should mention about that browsers may not render the caret with the specified style while there is composition string. Otherwise, if the browser respects the specified style, some IME users may complain about the composition string is hard to read or just ugly, or if the browser does not respect the specified style for composition string, some web developers may complain about the browser's behavior.

Cc: @dholbert, @emilio, @miketaylr, @zcorpan