Stack Exchange doesn't support the table tag, so if we want to present tabular data, we're supposed to do it using pre-formatted text. Unfortunately, it doesn't seem to work too well with Japanese; most Japanese characters are supposed to be twice the width of Latin characters, but on this site, they're slightly thinner than that. As a result, the columns fall out of alignment and tables become unreadable.
Here's what it looks like in Chrome when I enter in some text, using all single-width spaces:
The columns gradually veer to the left, making tabulated data hard to read.
I think this problem occurs because PRE and CODE blocks are styled with a font that doesn't include Japanese characters, so browsers have to borrow glyphs from Japanese fonts to render the missing code points. If I append the following to the site CSS as a workaround:
...the problem vanishes for me and I'm able to present tabular data in a readable format. Notice how in the following image, the Latin characters are obviously different, but the Japanese still looks the same. This is because the system was already borrowing the glyphs from the font I selected (MS Gothic).
This is what it looks like with modified CSS:
I'm not a web designer, and I can't say for sure which fonts should go on the list. (Please see the comments for some discussion of potential fonts.) But hopefully this question is enough to demonstrate why the problem occurs, and hopefully a web designer can figure out how to solve it without causing readability to suffer.
For reference, here is the text I entered for my examples:
This is a table, made by hand using pre-formatted text.
col1 col2 col3
blahblah blahblah blahblah
morestuff morestuff morestuff
猫 猫 猫
日本語で 日本語で 日本語で
日本語で日本語で 日本語で日本語で 日本語で日本語で