- Code point is sometimes not enough to determine the glyph. For example, U+5199 must look different in Simplified Chinese and Japanese. Typically this is handled by using different fonts, but more formally it should be marked with different lang attributes (in case of HTML).
- Top-to-bottom writing mode is still pretty much in use in Japanese. HTML support is poor, but it's common in PDF. Caveat: Latin alphabets are rotated by 90 degree (as explained in OP), but for punctuations, simply rotating the glyph isn't enough because the center line is going to be slightly off. You need special glyphs for rotated punctuation.
- Unlike Latin scripts, most Chinese and Japanese characters are free to break line at any point, but there are exceptions: contracted letters (e.g. ちょっ) cannot be split. So you'll end up with treating each letter as a separate word. (Contracted letters are treated as one word.)