I have a piece of code, but it doesn't work as I expected. Could you tell me why?

[xiang-1208]

I have a piece of code, but it doesn't work as I expected. Can you tell me why? (Stupid though it is)

世界坐标系设定为 ${W}$，是一个在 ${L}$ 初始位置重合的 3D 坐标系。点 $i$，$i \in \cal{P}k$ 在 ${L}$ 中表示为 $\boldsymbol{X}{(k, i)}^{W}$。

It is shown as

[edemaine]

It looks like the two underscores (for subscripts) are getting interpreted by your Markdown parser as italics. You could escape them (use \_), or use a Markdown plugin that supports KaTeX directly.