In the Unicode Standard document, v. 13, it says on p. 877 (bold-facing mine):
Squared Katakana words are Katakana-spelled words that fill a single display cell (em-square) when intermixed with CJK ideographs. Likewise, the square Latin abbreviation symbols are designed to fill a single character position when mixed with CJK ideographs. Note that modern software for the East Asian market can often support the comparable functionality via styles that allow typesetting of arbitrary Katakana words or Latin abbreviations in an em-square. Such solutions are preferred when available, as they are not limited to specific lists of encoded symbols such as those in this block.
This sounds quite remarkable to me. Where or with what fonts and software can I try that out? (I’m completely new to this. Also, I don’t know if this is the right StackExchange forum to ask, but there is no typography related one and the others didn’t fit at all.)