A big part of this is due to Chinese variants on the same Japanese “unicode codepoint”. Although, much of other things is due to fonts themselves.
Also, not every similar Kanji are on the same unicode codepoint.
Personally, I don’t think every difference matters; but there are indeed too many differences that can matter, so I am creating this thread to allow comments for experiences / opinions.
Apparently, this doesn’t happen in Japan, or in Japanese non-digital media, anyway; so I think it is debatable whether you need to know about these variants - if you are already in Japan, or otherwise have no problems.
角 vs 角
- Japanese 角
- Simplified Chinese 角
In the character 角 in traditional Chinese, as well as in Japanese, Korean and Vietnamese, the central vertical stroke does not extend below the last horizontal line, the one formed by the seventh, last stroke. This creates an appearance roughly equivalent to 土 enclosed in the top part of 冂.
However, in simplified Chinese, the central vertical stroke in 角 extends to the bottom of the character (as in 用). This difference applies to all simplified Chinese characters containing this radical.
Note that the Chinese and Japanese stroke order is slightly different for the fifth and sixth strokes, as illustrated in the animations above.
冎 vs 冎 in 過ぎ, 骨
- Japanese 過
- Simplified Chinese 過
The top section of the character turns left (◱) in PRC and Singapore, while it turns right (◲) elsewhere.
直 vs 直
- Japanese 直
- Chinese 直
Note the different forms, which differ in two respects: both Simplified and Traditional do not have a left vertical stroke, while the orthodox form has a left vertical stroke; and in simplified form, the top component is connected with the bottom, while in the orthodox they are separated – this last difference is shared with unrelated characters 真 and 具. The older form and stroke order are officially used in Japan but an alternative writing in other countries.
誤 vs 誤 in 誤解
- Japanese 誤
- Simplified? Chinese 誤
- Traditional Chinese 誤
This character, 吴, is the simplified and variant traditional form of 吳.
極 vs 極
- Japanese 極
- Simplified? Chinese 極
- Real simplified Chinese form is actually 极
今 vs 今
- Japanese 今
- Simplified Chinese 今
In all regions except mainland China, the stroke between 𠆢 and ㇇ for 今 (and its derived characters) is written with a horizontal 一 line.
In mainland China (based on Xin Zixing standardized form), the stroke between 𠆢 and ㇇ for 今 (and its derived characters) is written with a slanting 丶 dot instead.
In the historical Kangxi dictionary, the component below 𠆢 is written 一 followed by 𠃍 instead.
器 vs 器
- Japanese 器
- Chinese 器
In all regions except Japan, the middle component is written 犬, which is the orthodox form found in the Kangxi dictionary.
In Japanese shinjitai, the middle component is written 大 instead of 犬 (the upper right dot is missing). Due to Han unification, both forms are encoded using the same code point. This character may appear to be different depending on the font used.
A CJK compatibility ideograph exists at U+FA38 for the kyūjitai form of 器 which has 犬 as the middle component.
空 vs 空
- Japanese 空
- Chinese 空
In Chinese, the 5th stroke is straightened, but in Japanese, it is curved.
以 vs 以
- Japanese 以
- Chinese 以
以 (Kangxi radical 9, 人+2 in Chinese, 人+3 in Japanese, 4 strokes in Chinese, 5 strokes in Japanese.
画 vs 画
- Japanese 画
- Chinese 画
海 vs 海
- Japanese 海
- Chinese 海
These are often not really “font” differences, but usually, there are “forms” (i.e. 字体).
- 教科書体 / 明朝体
- 異体字 (許容字体)
In addition, there might be multiple ways to encode 噌.
- 教科書体 / 明朝体
- 異体字 (許容字体)
There are more to difficulty beyond forms, like blurring / pixelation, handwriting, fancy fonts (really “fonts”); font size, and font weight; but I am not exactly sure of the significance. There are extra small, and possibly blurry 振り仮名 too.
There are also Kanji choices, but the vocabulary with the alternative Kanji choice isn’t in a dictionary.